1451
Langmuir 1990,6, 1451-1454
Photochromic Behaviors of Long Alkyl Chain Spiropyrans at the Air-Water Interface and in LB Films Eiji Ando,’ Kumiko Moriyama, Koji Arita, and Kazuhisa Morimoto Central Research Laboratories, Matsushita Electric Industrial Co. Ltd., Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan Received August 10,1989. In Final Form: March 21, 1990 Systematic monolayer characteristics and photochromic behaviors of long alkyl chain spiropyran compounds were observed in a monomolecular layer at the air-water interface and in LB films. UVinduced merocyanines of 1’-octadecyl- (SP12), 1’- hexadecyl- (SP16), and l’-octadecyl-3’,3’-dimethyl-6nitrospiro[W-l-benzopyran-2,2’-indoline] (SP18)gave stable monolayers at the air-water interface. These monolayers changed from spiropyran (SP) to photomerocyanine (PMC) forms with UV irradiation for lower surface pressures and formed J aggregates with increase in surface pressure. Photochromic reactions including J aggregation were reversible at the airwater interface, and the half-decay periods of J aggregates were 103times longer than those of transferred f i i s . The formation of the aggregates had not been previously observed for the reason that J aggregates in LB films immediately returned to SP via the PMC form. This behavior is quite different from that of SP1822, which is the first J-aggregated photochromic compound observed. From measurement of the absorption with various angles of incidence of the polarized beam at the air-water interface, the change of anisotropy of the absorption in a layer plane was observed with increase in the formation of J aggregates depending on the surface pressure. The transition dipole moments of chromophores were randomly oriented on the subphase for lower surface pressures, corresponding to PMC form, and then took up a nearly flat orientation parallel to the direction of compression at higher surface pressures, corresponding to J aggregates. A model of molecular arrangements of long alkyl chain spiropyrans at the air-water interface in the process of the photochromic reactions, including the formation of J aggregates, is proposed. The molecular arrangement at the air-water interface suggests the formation and . . stability of J aggregates in LB films, and furthermore it indicates the difference for photochromic behaviors of the spiropyrans.
Introduction The Langmuir-Blodgett (LB) technique has proved to be a useful method for controlling and modifying photochromic reactions of organizing reactive molecules in intentionally high-order structures. The trans-to-cis conversion of (dihexyloxy)thioindigol in LB films did not occur while it is reversible in solution. N-Octadecyl-2 and N-hexadecylspiropyransM in LB films showed the ordinary photochromic reactions in the same way as the solutions. The first J aggregation of photochromic compounds was reported in the system of LB films composed of the mixture of SP1822 and o ~ t a d e c a n e . ~The ~ ’ absorption of the J aggregates was a sharp and intense band, and the halfdecay period in the dark was lo4 times longer than that of conventional spiropyrans. The next J aggregate was found in the system of LB films composed of a mixture of SP1801 and stearic acid.s We proposed a multifrequency optical memory device constructed with a layered structure of LB films on the J-aggregated photochromic compounds having different sharp absorption bands.g There has been an interest in the possibility of controlling (1) Whitten, D. G. J . Am. Chem. SOC. 1974,96,594. ( 2 ) Polymeropoulos, E. E.; Mobius, D. Ber. Bunsen-Ges. Phys. Chem. 1979,83, 1215. ( 3 )Morin, M.; Leblanc, R. M.; Gruda, I. Can. J. Chem. 1980,58,2038. (4) McArdle, C. B.; Barraud, A,; Ruaudel-Teixier,A. Thin Solid Films 1983, 99, 181. (5) Holden, D. A.; Ringedorf,H.; Deblauwe, V.; Smeta, G. J.Phys. Chem. 1984.88.716. (6j Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Thrn Solid Film 1985,133, 21. (7) Seki, T.; Ichimura, K.; Ando, E. Langmuir 1988, 4, 1068. (8)Ando, E.; Hibino, J.; Hashida, T.; Morimoto, K. Thin Solid Films 1988,160,279. (9) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Proc. Int. Srmp. on Future Electron Devices: November 20-21. 1985: Research &d DeveloDment Association for Future Electron Devices: Tokyo, p 47.
0743-7463/90/2406-1451SO2.50 -~ -~ ,10. I
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SP1822
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CH2OCOC21 H43
Figure 1. Chemical structures of long alkyl chain spiropyrans.
the photochromic reaction and the mechanism of formation of aggregates. In the present work, systematic monolayer characteristics and photochromic behaviors of SP1, SP7, SP12, SP16, and SP18, as shown in Figure 1,a t the air-water interface and in LB films have been investigated. The processes of the formation of J aggregates in a monomolecular layer at the air-water interface were observed, and the dichroic ratios were determined by measuring the absorption with various angles of incidence of polarized beam. The arrangement of molecules and stability of J aggregates on the subphase and LB films were discussed.
Experimental Section The synthesis of long alkyl chain spiropyran, 1’-hexadecyl3,3’-dimethyl-6-nitrospiro- [ 2H- l-benzopyran-2,2’-indoline] (SP16)was described in our previous study? 1’-Methyl-(SPl), 1’-heptyl- (SP7), 1’-dodecyl- (SP12), and 1’-octadecyl-3’,3‘dimethyl-6-nitrospiro[2H-l-benzo-pyran-2,2’-indoline] (SP18) 0 1990 American Chemical Society
1452 Langmuir, Vol. 6, No. 9, 1990
Ando et al.
IB
I* -=9
clld
i- f '
C
e'
C
A R E A (nmZ/molecule)
e
direction of compression
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Figure 2. Scheme for measurements of absorption: (A) a monolayer on subphase; (B)a transferred monolayer on a quartz glass slide; (C) a monolayer on a subphase with a beam polarized by a polarizer (p). A ray of the light incident (c) at an incident angle (0) from light source (9) is passed through a monolayer (b) on the subphase (e) or a slide (e'), and a reflected ray (d) from a mirror (a) or a transparent ray (d') is detected with photodiodes (f or f') in a fiber. were synthesized according to the same method. Field desorption mass spectrometry spectra of SP1, SP7, SP12, SP16, and SP18 showed peaks of a molecular ion M+ at m / e = 322, 406, 476, 532, and 560, respectively. Measurements of the surface pressure-area ( P A ) isotherms of monolayers at the air-water interface, the film balance used, and procedures for the formation of monolayers were described in the previous study.6 Monolayers were prepared from IOb3M solutions of the spiropyrans in spectroscopic grade benzene. The spiropyrans on the subphase were irradiated with 100 WJcm-* by UV lamps (wavelength 366 nm, Ultraviolet Products, UVL-56, USA) all during the spreading and the compression. Substrates on a hydrophobic quartz glass slide were precoated with trimethylchlorosilane in toluenes after cleaning with chromic acid mixture and rinsing many times; substrates on a hydrophilic slide were bombarded with plasma. The absorption spectra of monolayers were measured by multichannel photodetector luminescence spectroscopy (Otsuka Electronics, MCPD-100, Japan), with a spectral resolution of 1 nm. Figure 2 shows an experimental apparatus for absorption measurements of the monolayers at the air-water interface (A) and of the transferred monolayers to quartz slides (B). Dichroic ratios were determined by the absorption measurements of monolayers with various incident angles of polarized beam, as shown in Figure 2C. The reflection of an air-monolayer or a monolayer-water interface was not corrected, because the position of the photodetector was installed to avoid the optical passes of reflected rays of an air-monolayer and a monolayerwater interface. The intense ratios of reflected rays of an airmonolayer and a monolayer-water interface to total reflected rays were 14% and 18% at 620 nm, respectively.1°
Results and Discussion 1. Monolayer Characteristics. Figure 3 shows a-A isotherms for SP1, SP7, SP12, SP16, and SP18 at the airwater interface. For SP1 and SP7, monolayers were not obtained, with or without UV irradiation. Spiropyran (SP) forms of SP12, SP16, and SP18 did not form monolayers on the subphase in the dark, while photomerocyanine (PMC)forms resulting from UV irradiation of SP forms gave stable monolayers. This is probably because the (10)Collins-Gold, L.; Mobius, D.; Whitten, D. G. Langmuir 1986, 2,
191.
Figure 3. 7 - A isotherm for SP18, SP16, SP12, SP7, and SPI with (solid lines) and without (dotted lines) UV irradiation. compounds crystallize or are expelled from the monolayer in the dark. Our results for SP16 and SP18 agree with those of Holden a t and Polymeropoulous et a1.,2 respectively. The isotherms indicate that SP12 and SP16 are condensed types corresponding to the closest packed chromophores inclined, and SP18 corresponds to the closest packed alkyl chains from the molecular model. We observed that the PMC forms of SP12, SP16, and SP18 showed better spreading behavior compared with the SP forms when the spiropyrans were spread on the subphase. It is suggested that the hydrophobicity of the alkyl chain balances the hydrophilicity of the zwitterionic chromophore in a molecule under UV irradiation for SP12, SP16,and SP18. SP1 and SP7 have weak hydrophobicity because their alkyl chains are shorter than those of SP12, SP16 and SP18. Therefore, SP1 may be soluble in the subphase as indicated by the failure to detect an absorption of a PMC form under UV irradiation, while SP7 may crystalline or be expelled because of the presence of the absorption of the PMC form. Addition of hexadecane (HC18) to spiropyrans, SP12, SP16,and SP18, had no effect on the P A isotherms, while SP1822 and SP1801 with hexadecane gave a condensed monolayer in a reasonable molar ratio.6*8It is considered that the molecules of HC18 fit in the geometrical spaces of chromophore rings of SP1822 or SP1801, which have a flat orientation in the layer plane,ll while they do not fit in those of the PMC forms of SP12, SP16, or SP18, which are randomly oriented in the plane of the interface, as described in the following polarization data. 2. Photochromic Behaviors. Figure 4 shows the absorption spectra for the monolayer of SP16 at different surface pressures; SP12 and SP18 gave similar spectra. The change of the absorption spectrum of SP7 was not clear because SP7 did not form a stable monolayer. SP16 showed the PMC form (A, = 550 nm) for lower surface pressures of about 0-10 mN/m and at molecular area above 0.45 nm2. However, it exhibited an absorption spectrum different from that of a normal PMC form for higher surface pressures of 10-25 mN/m and a t molecular area below 0.4 nm2. By comparison with our results for SP18226 and SP1801,8this suggests gradual replacement of monomeric PMCs by J aggregates (J-PMC;A, = 618 nm). When the irradiation was stopped at 25 mN/m, the J-PMC decreased very slowly, and the half-decay period of J-PMC on the subphase was approximately 104s for SP16. When irradiation was resumed, J-PMC again increased. SP16 showed the reversible photochromic reaction including J aggregation at the air-water interface, but the monolayer was stable for only 1or 2 h, after which the monolayer collapsed. (11)Ando, E.; Suzuki, M.; Moriyama, K.; Morimoto, K. Thin Solid Films 1989,178, 103.
Langmuir, Vol. 6, No. 9, 1990 1453
Spiropyrans at the Air-Water Interface
T
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Figure 4. Absorption spectra for a monolayer of SP16 at different surface pressures: a, 0; b, 5;c, 10;d, 15; e, 20; f, 25 mN/m on the subphase. 0.06
7
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after deposition.
Furthermore, the absorption spectra in the present system changed rapidly with time as soon as the monolayer was transferred to hydrophilic and hydrophobic q u a r t z slides a t 25 m N / m . Simultaneously with transferring the monolayer to the slide, the peak of J-PMC which formed on the subphase decreased and then extinguished, as shown in Figure 5. From the analysis of the decay of the absorptions in the dark, the absorbances of wavelengths from 530 to 590 nm corresponding to t h e PMC form decreased slowly, while those a t wavelengths from 620 to 640 nm corresponding to the J-PMC decreased rapidly. Figure 6 shows the decay curves of absorbance a t 620 nm. T h e decay rate of 0-60 s corresponding to a decrease of J-PMC was fast, and that of over 60 s corresponding to a decrease of PMC was slow. This may be attributed to return from J-PMC to SP through the PMC form. The half-decay periods of PMCs and J-PMCs were approximately 4 X lo2and 6 X 10 s for the hydrophilic slide and 2 X lo2 and 4 X 10 s for the hydrophobic slide, respectively. The stability of J-PMC on transferred films was 10-3 times shorter than that in the monolayer on the subphase. The SP form which returned from J-PMC on the slide changed to the PMC form under UV irradiation but did not form J-PMC again. Because of this difficulty in stabilizing J aggregates in LB films, the absorption (A, = 618 nm) could not be assigned to a J aggregate by fluorescence measurements. We assume that earlier studies2*5*6 that did not report J aggregates involved absorption spectral measurements over a few minutes after deposition. This behavior for SP16 was quite
WAVELENGTH ( n m )
Figure 7. Polarized absorption spectra for a monolayer of SP16 at incident angles: a, 0'; b, 30'; c, 45'; d, 60'.
different from the behavior of monolayers of SP1822sand SP18Ol8on the slides. 3. M o l e c u l a r A r r a n g e m e n t s . T h e molecular arrangement or orientation in the process of the formation of J aggregates of SP16 on the subphase was evaluated by measuring the absorption spectra with various incident angles of polarized beam, as shown in Figure 7. Figure 8 shows the dichroic ratios (PIS), which are defined as the ratio of absorbance with the polarized beam component parallel to the incident plane (P)to absorbance with the polarized beam component perpendicular to the incident plane (S).The optical incident plane was installed parallel to the direction of compression, perpendicular t o the moving barrier. With increase in the formation of J aggregates depending on the surface pressure, the change of anisotropy of the absorption in the layer plane a t the air-water interface was observed for SP12, SP16,and SP18. For lower surface pressures, the transition dipole momenta of the chromophores of the PMC forms are randomly oriented on the subphase. For higher surface pressures, the transition dipole moments of the chromophores take up a nearly flat orientation and are arranged parallel to
Ando et al.
1454 Langmuir, Vol. 6, No. 9, 1990 d i r e c t i o n of compression
*
-
0
0
5
10
15
20
25
SURFACE PRESSURE("/m)
Figure 8. Dichroic ratio (P/S) vs surface pressure for SP16 at different incident angles of polarized beam. Angles corresponding to the lines are also written in the figure.
the direction of compression in the layer plane. The orientation of transition moment in the plane of interface was calculated from dichroic ratios according to the method of Vandevyver.I2 The transition dipole moment formed an angle of 40-30' with the plane of the interface for PMC form and nearly 0' for the J-PMC form. They were obtained by using refractive indexes of 1 for air, 1.5 for a monolayer, and 1.334 for a subphase. The transition dipole moment of spiropyran coincides nearly with the long axis of chromophore of spiropyran. Consequently, the model of molecular arrangement of SP16 on the subphase is proposed in Figure 9 from the results of the *-A isotherm and the polarization data. It is generally known that oblong molecules such as cyanine dyesI3 arrange perpendicular to the direction of compression in the layer plane when they are compressed on the subphase. SP16, however, has a novel arrangement to the contrary. Furthermore, the system shows longrange order because the polarized absorptions were observed in any position of the monolayer on the subphase. This molecular arrangement is apparently not present on a solid substrate. This molecular arrangement of SP16 may be stable in the fluid monolayer on the sub(12) Vandevyver, M.; Barraud, A.; Rauder-Teixier, A. J. Colloid Interface Sci. 1982, 85, 571. Chollet, P.-A. Thin Solid Films 1978,52, 343. (13)Yonezawa,Y.;Mobius, D.; Kuhn, H. Ber. Bunsen-Ges. Phys.Chem. 1986,90,1183.
PMC --c J-PMC Figure 9. Bird's-eye views (the upper parts) and sectional planes (the lower parts) of the molecular arrangements in the process of the formation of PMC (in the lower surface pressures) and J-PMC (in the higher surface pressures) forms: (0and chromophore ring; (0and -) long alkyl chain of SP16 on the subphase. phase but unstable in the rigid transferred monolayer on the substrate. The molecular arrangement and orientation a t the air-water interface imply the stability of the formation of J aggregates in LB films. 0)
Conclusion UV-induced merocyanines of SP12, SP16, and SP18gave stable monolayers. They formed the stable J aggregates on the subphase and showed reversible photochromic reaction including J aggregation, for which the halfdecay period was lo3 times longer than those in LB films. The model of molecular arrangements of long alkyl spire pyrans at the air-water interface in the process of the photochromic reactions including the formation of J aggregates is presented. The transition dipole moments of the chromophores of J aggregates take up a nearly flat orientation of the interface and are arranged parallel to the direction of compression. I t is suggested t h a t the molecular arrangements for J aggregation on the subphase determine the possibility or stability of formation of J aggregation in LB films. Acknowledgment. This work was performed as a part of R & D of Basic Technology for Future Industries sponsored by New Energy Development Organization (NEDO). We thank Professor Tohru Takenaka and Dr. Junzo Umemura of Kyoto University for fruitful suggestions and discussions.
