Langmuir 1995,11, 643-649
643
Effects of Molecular Area on the Polymerization and Thermochromism of Langmuir-Blodgett Films of Cd2+ Salts of 5,7=DiacetylenesStudied Using UV-Visible Spectroscopy Alice A. Deckert," Jennifer C. Home,? Barry Valentine,$Lisa Kernan,$ and Lara Fallon" Department of Chemistry, College of the Holy Cross, Worcester, Massachusetts 01610 Received August 9, 1994. I n Final Form: November 9, 1994@ The first study of the polymerization and thermochromism in Langmuir-Blodgett films of the Cd2+salts of 5,7-docosadiynoic acid (DCDA) and 5,7-tretracosadiynoicacid (TTCDA) formed at molecular areas of 22 and 26 A2/moleculeis reported. The polymerization behavior and subsequent thermochromism of the resulting polymers was followed using visible spectroscopy. The high-density (low molecular area) polymerized films resulted in visible absorption spectra which were distinctly different from the spectra resulting from the low-density(high molecular area) polymerized films. The temperature dependence of the visible absorption spectra of the two densities of films was distinctly different as well. In addition, the temperature dependence of the visible absorptionspectra of the polymerized films could be qualitatively accounted for by a two-reaction scheme in which the blue (B) form of the polymer forms an intermediate purple (P)form in a reversible reaction. The purple form of the polymer is transformed to the red (R) form in an irreversible reaction. (B f P, P R). Estimated activation barriers for the forward reaction B P were obtained from the temperature dependence of the visible absorption spectra using a normal preexponential factor of 10l2s-l. For LB films of DCDA these were E = 22.4 and 21.7 kcal/mol for the low- and high-density films, respectively. For LB films of TTCDA these were E = 23 and 21 kcal/mol for the low- and high-density films, respectively.
-
-
Introduction Langmuir-Blodgett (LB) films provide a reproducible and experimentally simple method for producing organized media. Indeed LB films have been utilized for everything from model membranes' to dielectric spacers in studies of energy t r a n ~ f e rThese . ~ ~ ~highly ordered films provide a unique opportunity for studying reactions in organized media. Many reactions occurring in LB films have been observed and reported in the l i t e r a t ~ r e . ~ - l '
* Author to whom correspondence should be addressed. Current address: Department of Chemistry, Michigan State University, East Lansing, MI 48824. f Current address: Georgetown Medical School, Washington, DC 20057. 0 Current address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. Current address: Department of Chemistry, Duke University, Durham, NC 27706. Abstract published in Advance ACS Abstracts, January 1,1995. (1) Nikolelis, D.; Brennan, J.; Brown, R.; McGibbon, G.; Kroll, U. @
Analyst 1991, 116, 122. (2) Arden, W.; Fromherz, P. Ber. Bunsenges. Phys. Chem. 1978,82, 868. (3)Yamazaki, I.; Tamai, N.; Yamazaki, T.; Murakami, A,; Mimuro, M.; Fujita, Y. J . Phys. Chem. 1988,92, 5035. (4) Barraud, A,; Rosilio, C.; Ruaudel-Teixier, A. Thin Solid Films 1980, 68, 7. (5) Bubeck, C. Thin Solid Films, 1988, 160, 1. ( 6 ) Ruaudel-Teixier, A.; Leloup, J.;Barraud,A. Mol. C y s t . Liq. C y s t . 1986,134,347.
(7)Whitten, D. G.; Eaker, D. W.; Horsey, B. E.; Schmehl, R. H.; Worsham, P. R. Ber. Bunsenges. Phys. Chem. 1978,82, 858. (8) Banejie, A,; Lando, J. B. Thin Solid Films 1980, 68, 67. (9) Barraud, A,; Rosilio, C.; Ruaudel-Teixier, A. J . Colloid Interface
Sci. 1977, 62, 509. (10) Laschewsky, A.; Ringsdorf, H.; Schmidt, G. Thins Solid Films 1985,134, 153. (11) Rosilio, C.; Ruaudel-Teixier, A. J . Polym. Sci. 1975, 13, 2459. (12) Ahmad, J.; Astin, K. B. Langmuir 1988, 4, 780. (13) Valenty, S. J. Macromolecules 1978, 11, 1221. (14) Koch, H.; Laschewsky, A.; Ringsdorg, H.; Teng, K. Makromol. Chem. 1986,187, 1843. (15) Tieke, B.; Wegner, G. In Topics in Surface Chemistry; Kay, E., Bagus, P. S., Eds.; Plenum Press: New York, 1978.
0743-746319512411-0643$09.00/0
Unfortunately, LB films have been underutilized for kinetic or mechanistic studies of reactions in organized media.12,17-21 The inherently low number of molecules in an LB film, coupled with the low sensitivity of most techniques used to obtain concentrations of species, leads to long data collection times. The long times typically needed for data collection make time-dependent data very difficult to obtain. Thus, the small number of kinetic studies in LB films is not surprising. The photopolymerization and subsequent thermochromism in LB films of the various diacetylenes of the form, RC=CC=C(CH2),COOH is well d o c ~ m e n t e d . ' ~ - ~ ~ __
(16) Rabe, J. P.; Rabolt, J. F.; Brown, C. A.; Swalen, J. D. Thin Solid Films 1986. 133. 153. ( 1 7 ) L d u b e r ,L. A.; Scheunemann, U.; Mohwald, H. Chem. Phys. Lett. 1986, 124, 561. (18) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991, 7, 2336. (19) Hasegawa, T.; Ishikawa, K.; Kanetake, T.; Koda, T.; Takeda, K.; Kobayashi, H.; Kubodera, K. Chem. Phys. Lett. 1990, 171, 239. (20) Koshihara, S.; Tokura, Y.; Takeda, K.; Koda, T.; Kobayashi, A. J . Chem. Phys. 1990,92, 7581. (21) Deckert, A. A.; Fallon, L.; Kiernan, L.; Cashin, C.; Perrone, A.; Encalarde, T. Langmuir 1994, 10, 1948. (22) Ogawa, K.;Tamura, H.; Hatada,M.; Ishihara, T.Langmuir 1988, 4. 903. (23) Ogawa, K. J . Phys. Chem. 1989,93, 5305. (24) Tieke, B.; Bloor, D. Mukromol. Chem. 1979,180, 2275. (25)Kaneko, F.; Dresselhaus, M. S.; Rubner, M. F.; Shibata, M.; Kobayashi, S. Thin Solid Films 1988,160, 327. (26)Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983,99, 95. (27) Kajzar, F.; Messier, J. Thin Solid Films 1983, 99, 109. (28) Tamura, H.; Mino, N.; Ogawa, K. Thin Solid Films 1989,179, 33. (29) Fischetti, R. F.; Filipkowski, M.; Gerito, A. F.; Blasie, J. K Phys. Rev. B 1988,37, 4714. (30) Mivano. K.: Maeda. T. Phvs. Rev. B 1986. 33. 4386. (31) Nikahara, 'H.; Fuk'uda, K.; Seki, K.; Asada, 'S.;Inokuchi, H. Chem. Phys. 1987, 118, 123. (32) Lyall, I. R. J.; Batchelder, D. Br. Polym. J. 1985, 17, 372. (33) Eckhardt, H.; Baudreaux, D. S.; Chance, R. R. J . Chem. Phys. 1986,85, 4116. (34) Wenzel, M.; Atkinson, G. H. J.Am. Chem. SOC.1989,111,6123. (35) Chollet, P. A.; Kajzar, F.; Messier, J. Thin Solid Films 1985, 132, 1.
0 1995 American Chemical Society
Deckert et al.
644 Langmuir, Vol. 11, No. 2, 1995 These diacetylenes undergo rapid polymerization when exposed t o W light (-254 nm). The polydiacetylene formed has a rigid, one-dimensional, conjugated backbone. This conjugation results in a strong n to n* absorption in the visible region. Due to the intensity of the visible absorption spectrum, short data collection times are possible and the time dependence of the thermochromism can be followed using simple transmission visible absorption spectroscopy. A wide variety of diacetylenes with the general formula RC=CC=C(CHz),COOH have been studied in LB films. All of these molecules undergo rapid photochemical polymerization when exposed to U V light a t -254 nm.17-42 This polymerization is apparent in both LB films and the solid state as a color ~ h a n g e . l ' - ~Most ~ commonly the polymer is royal blue in color whereas the monomer is colorless. The blue polymer undergoes a thermochromic change common to all polydiacetylenes and becomes bright red upon heating.l5 This chromism can also be initiated by solvents such as chloroform or ethanol or by exposure to UV light.15 As the value of n decreases and the diacetylene group is moved closer to the acid head group, the solid state polymerization proceeds rapidly from the blue to the red form of the polymer during UV irradiaAlthough this behavior has been documented for solid state 5,7-diacetylenes, and films of 2,4-diacetylenes, no LB films of 5,7-diacetylenecarboxylicacids have been reported to date. Although some controversy still exists in the litera~ u ~ ~ , ~theJ most ~ J widely ~ J ~accepted , ~ ~ explanation for the thermochromism is a phase change from a monoclinic to a n orthorhombic arrangement of the side chains R and (CH2),COOH.29 This rearrangement of the side chains causes a stress to the conjugated backbone of the polymer resulting in a shift of the n and n* electronic energy level^.^^,^^
The thermochromic change in polydiacetylenes has been shown to be reversible.1s-21 The degree of reversibility has been connected to the degree of disorder in the polymer side chains by a number of authors.18-21 The reversibility has also been shown to be affected by the molecular area of the layers.26z28The current investigation provides a n in-depth study of the thermochromism in LB films of the Cd2+ salts of 5,7-docosadiynoic acid (DCDA) and 5,7tetracosadiynoic acid OY"I'TDA)a t two different molecular areas of 22 and 26A2/molecule. Langmuir-Blodgett films of DCDA and TTCDA were built up on glass substrates. These films were then polymerized to give very different visible absorption spectra depending on the density ofthe film. The thermochromism of the polymer films was then investigated using visible spectroscopy.
Experimental Section Synthesis of S,7-Docosadiynoic Acid and S,7-Tetracosadiynoic Acid. The syntheses of 5,7-docosadiynoic acid and 5,7-tetracosadiynoic acid were carried out by the following general scheme: (36)Berkovic, G.; Superfine, R.; Guyot-Sionnest, P.; Shen, Y. R.; Prasad, P. N. J . Opt. SOC.Am. B 1988,5,668. (37)Sauteret, C.; Hermann, J. P.; Frey, R.; Pradere, F.; Ducuing, J.; Baughman, R. H.;Chance, R. R. Phys. Reu. Lett. 1976,36,956. (38)Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980,68,77. (39)Tieke, B.; Lieser, G.; Wegner, G. J . Polym. Sci. 1979,17,1631. (40)Bloor, D.; Ando, D. J.; Obhi, J. S. Makromol. Chem. Rapid
C,H2,+1C=CI
Cu+,C,H,O, *
CnH2,+,C~CC=C(CH2)3COOH
The first step used the Grignard reagent of butyl bromide as a strong base to remove the acetylenic proton from a terminal alkyne (1-hexadecyneor 1-octadecyne). The resulting carbanion was then reacted with molecular iodine to yield the l-iodo-lalkyne in good yield.35,43-45The iodoalkyne was purified by cryogenic distillation on a mercury condenser vacuum line. A total of five freeze-pump-thaw cycles resulted in a light yellow, clear liquid which was identified as pure 1-iodo-1-hexadecyne (1-iodo-1-octadecyne)by 1H NMR. The purified iodoalkyne was coupled to 5-hexynoic acid by the Chodkievicz reaction46to give the 5,7-diacetylene in -25% yield. The purity of the resulting product after recrystallization from petroleum ether was verified using lH NMR. The 5,7-docosadiynoic acid gave a melting point of 72.5 "C while the 5,7-tetracosadiynoic acid gave a melting point of 78 "C. The Grignard reaction was carried out in a dry three-necked flask equipped with a dropping funnel, a condenser, and a drying tube. A 1.7 g sample of magnesium ribbon, 7 mL of butyl bromide, and 15mL of ether were warmed slightly to initiate the Grignard reaction, which was then allowed to proceed unheated under reflux for 5 min. A 13 mL sample of 1-hexadecyne (14 mL of 1-octadecyne) and 13mL of ether were combined in the dropping funnel and added to the reaction mixture dropwise over a period of 30 min. The reaction was allowed to reflux for 1 h. Approximately 15 g of iodine was added through the condenser to the gray reaction mixture until a dark maroon color persisted. The reaction mixture was transferred to a 500 mL beaker and the reaction was quenched carefully with 100 mL of distilled water. Acidification with 20 mL of acetic acid resulted in a red solution. The ether layer was separated, and the aqueous layer was extracted three times with small portions of ether. The ether extracts were combined with the ether layer and washed twice with 15 mL of 10% sodium thiosulfate solution. The ether layer was dried over MgS04, and the ether was removed by rotary evaporation. The crude iodoalkyne was a reddish-brown liquid. This was further purified by distillation on the mercury condenser vacuum line to produce 4.5 g of light yellow clear liquid l-iodo1-hexadecyne (5 g of 1-iodo-1-octadecyne). The pure iodoalkynes were coupled to 5-hexynoic acid in the following way. A solution of 3 mL of 5-hexynoic acid and 7 mL of 10% NaOH was added to a 250 mL two-necked flask equipped with a dropping funnel and a stopper. The flask was maintained at 15 "C by means of a water bath. A mixture of 0.615 g of hydroxylamine hydrochloride and 0.303 g of CuCl in 4 mL of ethylamine was added to the clear yellow mixture resulting in a cloudy yellow color. After being stirred at 15 "C for 15 min, the 4.5 g of 1-iodo-1-hexadecyne (5 g of 1-iodo-1-octadecyne)in 40 mL of methanol was added dropwise to the reaction mixture. If any green-blue color was observed (indicative of Cu2+),a small amount ofhydroxylamine hydrochloride was added to regenerate the Cu+. Approximately 5 min after all the 1-iodo-1-alkyne had been added, the reaction was removed from the ice bath and acidified with 35 mL of 2 N HzS04. The product was extracted into ether. The ether layer was dried over MgS04, and the ether was removed by rotary evaporation. The white crystals were recrystallized from petroleum ether and identified as 5,7-DCDA (5,7-TTCDA) using 'H NMR and IR spectroscopy. In addition, exposure to U V light resulted in rapid color changes from white to blue to red. Preparation of Langmuir-Blodgett Mdtilayers. The preparation of the LB multilayers was accomplished by use of the well-documented Langmuir-Blodgett dipping m e t h ~ d . ~ ~ ~ ~ ~
(43)Vaughn, T.H. J . Am. Chem. SOC.1933,55,3453. (44)Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. angew. Chem., Int. Ed. Engl. 1976,15,764. (45)Yoshioka,Y.; Nakahara, H.; Fukuda, K. Thin Solid Films 1986, 133,11. (46)Viehe, H. G., Ed. Chemistry of Acetylenes; Marcel Dekker Inc.: Commun. 1986,7,665. (41)Okada,S.;Nakanishi,H.;Matsuda,H.;Kato,M.;Sugi,M.;Saito,New York, 1969;Chapter 9. M.; Iizima, S.Polym. J . 1988,20,493. (47)Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937,51,964. (42)Saito, K.;Saito, M.; Ikegami, K.; Kuroda, S.; Sugi,M. Jpn. J . (48)Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Appl. Phys. 19SS,27,1038. Interscience Publishers: New York, 1966.
Langmuir-Blodgett Films of Cd2+Salts The synthesized DCDA and TTCDA were recrystallized from petroleum ether (bp 50-110 "C) immediately prior to volumetric preparation of a -1 mg/mL solution in spectral grade chloroform. A microsyringe was used to spread 150 p L of solution dropwise over the subphase surface. The subphase consisted of ultrapure water from a Millipore reverse osmosis still with deionizing filters. The water had a resistivity greater than 17 MQ cm. Two types ofLB films ofthe cadmium(2+)poly-5,7-diacetylenes were observed when the films were transferred under different subphase conditions. These films, which had very different molecular areas, also differed in their visible absorption spectra. Films transferred at a surface pressure of 20 dyn/cm from a subphase having a Cd2+ concentration of 2.5 x M held at a temperature of 20 "C had molecular areas of -22 k . Films transferred at a surface pressure of 10 dyn/cm from a subphase having a Cd2+concentration of 8.2 x M held a t a temperature of 10 "C had molecular areas of -26 The subphase was prepared using cadmium chloride to achieve either a 2.5 x or 8.2 x 10-6 M solution of Cd2+ in ultrapure water. For either subphase, the solution was brought to a pH of -6.5 using 0.01 M NaHC03. The entire LB trough system was enclosed in a laminar flow hood providing class 100 clean air. After spreading the DCDA or TTCDA monolayer, the Langmuir films were compressed a t a rate of 20 cmVmin. A constant surface pressure was maintained throughout deposition ofthe multilayers. Thirty Y-type layers were deposited by passing the hydrophobic glass substrate through the airlayer-water or water-layer-air interface a t a constant rate of 8 mm/min to produce one monomolecular layer for each pass through the interface. Transfer ratios between 80 and 100% were obtained for both types of DCDA and TTCDA films. The substrate consisted of a glass microscope slide which had been thoroughly cleaned with soap and water and subsequently sonicated for 15min each in spectral grade chloroform, isopropyl alcohol, subphase quality water, isopropyl alcohol, and subphase quality water in that order. The slides were then rendered hydrophobic by placing them in a beaker containing 3 drops/ slide of 1,1,1,3,3,3-hexamethyldisilizane. The beakerwas covered with a watch glass and let stand overnight. Polymerization of Multilayers. Polymerization of the 30 layer DCDA or TTCDA films was accomplished by exposure to a Hg arc lamp at a distance of 10 cm directly in front of the lamp aperture. The Hg lamp power output was measured using a ferrioxalate chemical a~tinometer.~g The spectral output of the Hg lamp was peaked a t 254 nm and was -99% 254 nm light. At a distance of 10 cm directly in front of the lamp, the photon flux was determined t o be 3 x 1017 photond(cm2 s). Assuming only 254 nm light, this photon flux corresponds to a power density of 230 mW/cm2. Both sides of the glass substrates containing the 30 layer DCDA or TTCDA LB films were exposed to the UV radiation from the Hg lamp for various times to determine the time necessary to accomplish complete conversion to blue polymer while minimizing any conversion to the red p01ymer.l~A time of 7 midside was found to be satisfactory for both types of DCDA. A polymerization time of 5.5 midside was used for both types of TTCDA films. UV-Visible Spectroscopy Experiments. The polymerization and subsequent thermochromism were followed by optical absorption using a Perkin Elmer Lambda 4c W-visible spectrometer in transmission mode. The reference beam of the double-beam instrument was transmitted through air. The repetitive scan feature of the spectrometer was utilized to obtain spectra every 2 min as the temperature of the film was changed. Temperature control of the film was accomplished with a custom-made slide holder described previously through which water from a thermostated bath was circulated.21 The temperature of the film was monitored using a type K chromelconstantan thermocouple attached with epoxy to the slide holder. Slide temperatures could be read to h0.5 K with this arrangement. The water bath could be cooled t o 276 K and heated to 368 K. This temperature range allowed slide temperatures between 283 and 358 K. A nearly linear heating rate a t the sample was achieved by applying a constant current to the bath heater coil. A constant cooling rate was obtained by disabling
Langmuir, Vol. 11, No. 2, 1995 645
-
2
E -20 a
(49) Hatchard, C. G.;Parker, C. A. Proc. R . Soc.LondonA 1966,235,
-
,
-
,
"
Az.
518.
I
€40
20
0 0'
10
20
30
40
Area per Molecule (Az ) Figure 1. Typical pressure vs area isotherms of (a) 5,7docosadiynoic acid and (b) 5,7-tetracosadiynoic acid on a water subphase a t the Cd2+concentrations and temperatures indicated on the figure. The subphase was adjusted t o a pH of 6.6 for all isotherms. Films were transferred at a surface pressures indicated by solid dots on the figure. the water bath heater and allowing the refrigeration unit to cool at maximum. Slide temperatures constant to h0.5 K could be maintained for up to 1 h.
Results Isotherms. Figure 1 shows typical pressure-area isotherms of DCDA and TTCDA obtained for the above conditions. A molecular area of -22 Azlmolecule was obtained a t a surface pressure of 20 dydcm when a DCDA layer was compressed at 20 cmz/min on a subphase consisting of 2.5 x M Cd2+a t a pH of 6.5 maintained a t 20 "C. A molecular area of -26 A2/molecule was obtained at a surface pressure of 10 dydcm when a DCDA layer was compressed a t 20 cm2/min on a subphase consisting of 8.2 x M Cd2+a t a pH of 6.5 maintained a t 10 "C. Similar conditions resulted in similar molecular areas for TTCDA. Similar trends in molecular area with [Cd2+]have been reported for other diacetylene films.15~22*23 Photopolymerization. Figure 2 shows the visible absorption spectra for both types of DCDA films a s a function of time exposed to the output of a Hg arc lamp. The 30 layer DCDA and TTCDA films were held a t a low temperature of 284 K during exposure in order to minimize any thermal conversion to the red form of the polymer film. Both sides ofthe glass slide were treated identically to ensure polymerization ofthe films on both sides. Thus, a time of 100 s indicates that both sides were exposed a t a distance of 10 cm directly in front of the Hg lamp for 100 S.
Comparison of Figure 2a and b shows that the films having different molecular areas (densities) polymerize to give distinctly different visible absorption spectra. The low-density film (26 &molecule) shows two distinct peaks a t approximately 600 and 650 nm. As the polymerization progresses, a third distinct peak around 670 nm appears and a shoulder a t -540 nm grows in. F o r very long polymerization times, the shoulder a t 540 nm begins to dominate the spectrum. The high-density film (22 Azl molecule) results in a polymer spectrum that has much less distinction between peaks. For short polymerization times, "peaks" centered a t approximately 650, 600, and 565 nm are discernible. As the polymerization progresses, these peaks grow together and result in one broad peak centered a t -540 nm with shoulders around 565 and 650
Deckert et al.
646 Langmuir, Vol. 11, No. 2, 1995 l a
I
a
b
3455 nm
1
38 0.8 %
P m
+
0.3t
0.6
.I
P
0.4
0.2 0 450
550 650 280 310 340 370 Wavelength (nm) Temperature (K)
Figure 4. (a)Visible spectrumof a 30 layer high-densityDCDA film polymerized for 420 s as a function of film temperature. Absorbances are normalized to the peak absorbanceat 455 nm. (b) Normalized absorbances at 455 and 630 nm as a function of temperature. Normalization factors used are discussed in the text.
b
a
c 500 nm
630
1 I
1
283 K
8
5 0.8
g
t
m
3
d 0.6
-.Ym 0
E 0.4 0
2
0.2
0 450
550 650 280 310 340 37 Wavelength (nm) Temperature (K)
Figure 3. (a)Visible spectrum of a 30 layer low-densityDCDA film polymerized for 420 s as a function of film temperature. Absorbances are normalized t o the peak absorbance at 540nm. (b) Normalized absorbances at 500 and 630 nm as a function of temperature. Normalization factors used are discussed in the text.
nm. Figure 2c and d shows the spectra of 30 layer TTCDA films polymerized for 5.5 min. The data in Figure 2 reveal that the spectra for the high-density TTCDA films are nearly identical to the spectra for the high-density DCDA films. Likewise, the spectra for the low-density TTCDA films are nearly identical to the spectra for the low-density DCDA films. The thermochromism of DCDA was studied in films that had been polymerized for 420 s. The thermochromism of TTCDA was studied in films that had been polymerized for 330 s. TemperatureDependenceof AbsorptionSpectra. As seen in Figure 3, the thermochromism in the DCDA films was easily followed using visible spectroscopy. The data in Figure 3a show the effect on the visible absorption spectra of a 30 layer, low-density (26 A2/molecule)DCDA film polymerized for 420 s as the temperature of the film was increased at a nearly linear heating rate of 0.04 Ws. Figure 4 shows analogous data for a 30 layer, high-density (22 A2/molecule)DCDA film polymerized for 420 s. The
data in Figure 4 were obtained a t a heating rate of 0.04 W s. The temperature dependence of the polymer spectrum for the low-density film was more obvious than the temperatnre dependence observed for the high-density films. After U V polymerization, the low-density films showed distinct peaks a t 630 and 600 nm decreased while two distinct peaks a t 540 and 500 nm emerged. In contrast, the high-density films showed only one distinct peak centered at 540 nm with a n indistinct shoulder near 650 nm. This spectrum was more consistent with the red form of the polymer. Upon heating the high-density films, the absorbance at 540 nm decreased while the absorbance a t 500 nm increased, In addition, the shoulder around 650 nm disappeared. The resulting "red" polymer spectrum had two fairly distinct peaks a t approximately 540 and 500 nm. The two spectra for the red polymer were very similar except that the peaks were not nearly so well defined in the high-density spectrum, and the intensity of the peaks a t 500 and 540 nm was reversed. The spectra as a function of temperature shown in Figures 3a and 4a did not exhibit an isosbestic point. The lack of isosbestic point ruled out any simple process involving only two absorbing species, such as the blue and red forms of the polymer. A more complex mechanism involving a t least three absorbing species must be postulated. This suggested more than one important step in the reaction pathway. The lack of a n isosbestic point is corroborated in Figures 3b and 4b, which show the normalized absorbance a t 630 and 500 nm as a function of temperature. For the wavelength associated with the blue polymer (630 nm), absorbances decreased with temperature. These absorbances were normalized according to (A - A,)/(A, - AJ. For the wavelength associated with the red form of the polymer (500 nm), the absorbance increased with temperature. In this case the function (A - A,)/(A, -A,) was used to normalize the absorbances. The information obtained from each wavelength in the visible absorption spectra was distinctly different. The normalized absorbances in Figures 3b and 4b were proportional to the two-dimensional concentration, or coverage, of the species in the film which absorbs a t the wavelength of interest. Thus, an inflection point in Figure 3b (or 4b) represented a point where the second derivative of the coverage with respect to temperature has gone to
Langmuir-Blodgett Films of Cd2+Salts
Langmuir, Vol. 11, No. 2, 1995 647
8
8E
1
I
a 281 K b 358 c 282
1
C
m
m +? 0.8
2 0.8 0
2 a 0.6 W
-w 0.4 .-
zb
0.2 1
- 0 450
I
\
0‘ 550
450
650
Wavelength (nm) Figure 5. Visible spectra of a 30 layer low-densityDCDAfilm polymerized for 420 s taken during a thermal cycle from 280 to 325 K. Absorbances are normalized to the peak absorbance
at 540 nm. The thermochromicchange observed upon heating is only partially reversed upon cooling. In particular,the peaks associated with the red polymer do not reverse upon cooling.
zero. Since the rate of reaction can be expressed as the first derivative of the coverage with respect to temperature multiplied by one over the heating rate, the temperature at the inflection point represented the temperature of maximum reaction rate. Given the temperature a t the inflection point of 338 K for the low-density DCDA films and 330 K for the highdensity DCDA films, and assuming first-order Arrhenius rate constants, a n estimation of the activation barrier for the process resulting in the loss of visible absorbance in the blue polymer region of the spectrum and increase in the absorbance in the red polymer region of the spectrum can be obtained. For a first-order process the rate, r = k[B], where [Bl stands for the coverage of blue polymer was the first-order Arrhenius in the film and k =Ae(-E’Rn rate constant. The inflection point occurred when drldT = 0 = k(d[BYdT) (dk/dT)[Bl. Differentiating, rearranging, and recognizing that dTldt = /3 = heating rate, led to
+
ARFIEB =
(1)
The activation barrier a t which these two functions were equal for the temperature of the inflection point was calculated by assuming a “normal” preexponential for a first-order process of 1012/s. This calculation gave a n activation barrier of 22.4 kcaYmo1 for the low-density DCDA data presented in Figure 3b. A similar calculation gave a n activation barrier of 21.7 kcaYmo1 for the highdensity DCDA data shown in Figure 4b. Similar analysis gave activation barriers of 23 and 21 kcaYmol for the lowdensity and high-density TTCDA films, respectively. Thermal Cycles. In order to observe any reversible thermochromism, thermal cycling experiments were performed. Figure 5 shows a typical set of absorption spectra resulting from a thermal cycle in which the temperature of the 30 layer, low-density (26 A21molecule) DCDA polymer film was increased a t a heating rate of 0.04 W s from 280 K to just before the inflection point, 325 K. The film temperature was then decreased a t a rate of 0.02 W s from 325 to 280 K. A significant decrease in the absorbance due to the blue form polymer a t 630 nm and a n increase in the absorbance due to the red form polymer at 500 nm occurred by the time the film had reached 325 K. Cooling of the film to 280 Kresulted in partial recovery of the peaks at 640 and 600 nm attributed to the blue form
550
650
Wavelength (nm) Figure 6. Visible spectra of the same 30 layer low-density DCDA film for which the data in Figure 5 were obtained as the film is subjected to a second thermal cycle from 280 to 360 K.
The themchromic change observed upon heating to 360 K is only negligibly reversed upon cooling.
polymer. However, the peaks at 555 and 510 nm attributed to the red form polymer did not significantly decrease upon cooling to 280 K. Figure 5 clearly demonstrates the partial reversibility of the thermochromism between 280 and 325 K. Similar results were obtained for 30 layer TTCDA films annealed between 280 and 317 K. Figure 6 shows typical results obtained when the same DCDA film was subjected to a thermal cycle between 280 and 360 K. The temperature of 360 K was high enough to cause complete conversion to the red form according to Figure 3a. As seen in Figure 6, increasing the temperature to 360 K after an initial cycle to 325 Kresulted in complete conversion ofthe spectra associated with the blue polymer to the spectra associated with the red polymer. Cooling the film to 280 K resulted in a shift in the red polymer spectrum, but no real decrease or simultaneous increase in the blue polymer spectrum. Similar results were obtained for the previously annealed TTCDA film. The partial, but not complete, reversibility of the thermochromism between 280 and 325 K for DCDA and 280 and 323 K for TTCDA suggested that the mechanism for the thermochromism consisted of two steps, one of which is reversible. Thus, one process acted as a sink for the red form polymer resulting in incomplete reversibility of the absorption spectra as a function of temperature. The same thermal cycling experiments were performed on 22 A2/moleculefilms of DCDA. As seen in Figure 7, annealing to 322 Kand cooling to 280 Kresulted in almost no reversal of the absorption spectrum. When the same film was subjected to a further temperature cycle up to 360 K and back to 280 K the spectrum shifted toward lower wavelengths but never developed two peaks as observed in Figure 4. Figure 8 shows that the cooling cycle resulted in a shift back toward higher wavelengths with a n almost imperceptible shoulder at about 540 nm.
Discussion and Conclusions Polymerization. Differences in the visible absorption spectrum of polydiacetylene LB films as a function of density have been Most studies have produced spectra that are consistent with polymerization directly to the red, monoclinicform of the polymer when films are formed a t higher densities, or when films are polymerized a t higher temperatures as suggested by the current i n v e ~ t i g a t i o n . ~ ~However, , ~ ~ , ~ ~ , one ~ ~ study provides evidence for a n entirely different polymerization
Deckert et al.
648 Langmuir, Vol.11, No.2, 1995 I.2
I
a 283K b 322
’
q 0.2
I
\bd
-0
\’
!
400
500 600 Wavelength (nm)
700
Figure7. Visible spectra of a 30 layer high density DCDA film polymerized for 420 s taken during a thermal cycle from 280 Kto 322 K. Absorbances are normalized to the peak absorbance at 455 nm. The subtle thermochromic change observed upon heating is hardly reversed upon cooling. 1.2 a 2 8 3 ~
8
li
S
b 357
c 283
i
f! 0.8 U
0.6
2 0.4
400
500 600 Wavelength (nm)
700
Figure 8. Visible spectra of the same 30 layer high density DCDA film for which the data in figure 7 were obtained taken as the film is subjected to a second thermal cycle from 280 to 360 K. The thermochromic change is even more subtle and hardly reverses at all upon cooling.
mechanism a t high densities resulting in a polyacetylene (as opposed to a polydiacetylene) backbone consisting of conjugated double bonds with anywhere from halfto none of the triple bonds remaining.23 In this study the highdensity polymer is referred to as a “B-type”polymer. The B-type polymer was only observed under X-irradiation, and no polyacetylene polymerization could be observed under W radiationz3 Thus the polyacetylene-type polymerization mechanism is ruled out in the present study because both types of spectra were observed after irradiation by W light. Proposed Model. The following kinetic model for the thermochromism is consistent with all the data presented. B*P k2
P-R The symbols B and P represent the blue form of the polymer and a n intermediate “purp1e”formof the polymer. The symbol R represents the red form of the polymer. Although no direct, spectroscopic evidence for the intermediate form of the polymer exists, compelling, indirect
evidence has led several authors to propose such a n intermediate.1s~24~40 The partial reversibility of the thermochromism, lack of isosbestic point, and the fact that the peaks in the visible spectrum associated with the red form of the polymer (R) do not reverse, whereas those for the blue form (B)do reverse between 280 and 325 K, all point to the existence of this intermediate. The fraction of red polymer present at the start of the temperature cycle, R,, cannot be assumed to be zero. Likewise, the fraction of blue and purple polymer present a t the start of each experiment is not known. The intermediate is proposed to be a less ordered form of the blue polymer.1s~21Any intermediate present would be expected to broaden the peaks attributed to the blue form of the polymer without resulting in new distinct absorption maxima.lg Thus the fraction of purple polymer present after UV irradiation cannot be determined from the spectra independently of the fraction of blue polymer present. Although distinct maxima in the visible absorption spectra can be assigned to the red and blue modifications of the polydiacetylene, these peaks are broad and cannot be resolved. Thus direct integration is not possible to obtain the fraction of R and B present after UV irradiation. This leads to a four-parameter fit for any quantitative modeling of the data. The four parameters required are two activation barriers and the fraction ofblue and red polymer initially present in the film. Because of the large number of parameters required, no quantitative fit to the data is attempted. The activation barriers derived earlier from the temperature dependence of the spectrum represent estimates for the kinetic barrier between the purple form of the polymer and the red form of the polymer. The slowest, rate-determining step of the mechanism determines the inflection point in Figures 4b and 5b. If the blue to purple step is rate determining, then as soon as purple is made, it would become red. No reversibility would be observed under these conditions. If, however, the purple to red step is rate limiting, low-temperature anneals would result in a significant amount of purple being formed without producing much red. Upon cooling, the purple will then reverse back to the blue form of the polymer consistent with the data in Figures 6 and 8. Comparisonto Previous Studies. Two models have been proposed to account for the reversibility of the thermochromism in 10,12-pentacosadiynoic acid (PCDA).18J1 One model predicts a two-step serial scheme that is identical to that proposed here for 5,7-DCDA and 5,7-!M’CDA.18 The second model involves a two-reaction parallel scheme in which one reaction was reversible. This second model quantitatively accounted for the temperature dependence of the thermochromism of both 10,12-PCDA and 10,12-TCDA.21 A parallel scheme was suggested because a very definite decrease in the absorbances due to the red form were observed during the cooling leg of a thermal cycle from 285 to 328 K.21 The possibility of a third reversible reaction between the blue and purple forms of the polymer was suggested as possible but not necessary to quantitatively fit the data.21 This suggests that a comprehensive, general reaction scheme to account for the thermochromism in most diacetylene polymer films is as follows: B=P
(1)
B Z R
(2)
P-R (3) The details ofthe temperature dependence of the visible absorption are intimately related to the relative magni-
Langmuir-Blodgett Films of Cd2+Salts
c 6. rc
oJ
0 0
Langmuir, Vol. 11, No. 2, 1995 649
0
0
PURPLE
BLUE
I
REO
Figure 9. Schematic diagram of the model discussed in the text.
tudes of the activation barriers for each step. The relative magnitude of the activation barriers for each step are affected by the position of the diacetylene group as well as the density of the films. For both densities (22 and 26 AVmolecule) of the 5,7-diacetylene films, step 2 is much slower than either step 1 or 3 and a two-step serial mechanism is observed. For PCDAfilms with a molecular area of -26 AVmolecule, step 2 is also much slower than steps 1 and 3, resulting in a the same two-step serial mechanism.18 In contrast, for the films of 10,12-PCDA and 10,12-TCDAwith molecular areas of -23 Az/molecule, step 1is much slower than steps 2 and 3, and a two-step parallel mechanism is observed.21 This combined model supports the same microscopic interpretation as either the serial or parallel model previously proposed. The blue form of the polymer has a well-ordered film with the side chains in all-trans, orthorhombic configuration.18,21,29,34 The purple form has a more disordered but still essentially orthorhombic arrangement of the side chains.18*21The red form has a monoclinic arrangement of side chains. 18,21,29,34 The thermochromism is a result of the side chains tilting, which produces a stress on the polymer backbone and displaces the x and x* levels.33 The thermochromism is reversible from B to P as long as the side chains disorder without becoming irreversibly tangled.18 The thermochromism is also reversible from B to R if the interactions between side chains are strong enough to hold the side chains in a n essentially all-trans configuration as they tilt from the orthorhombic to the monoclinic ~ t r u c t u r e . l ~The - ~ ~hypothesis that inter-side-chain interactions are essential to the observation of a reversible thermochromism is supported by previous results.25 Figure 9 shows a simple schematic representation of the microscopic model proposed. This microscopic model is supported by X-ray20j29 and small-angle x-ray scattering (SAXS)38experiments. In addition, similar orderdisorder models for the thermochromism have been proposed by other authors.18-20,24,25,28,29,34
The fact that two mechanisms have been proposed for the thermochromism in 10,12-PCDA films is consistent with the fact that these studies were done a t different [Cd2+land hence different densities. At lower densities, the side chains are further removed from one another so that interactions between them are not strong enough to observe any reaction 2 (B s R). The result is an apparent two-step serial mechanism.18 At higher densities, the side chains are close enough to allow van der Waals interactions such that reaction 2 is observed.21 In addition, the proximity of the side chains result in reaction 3 (P R) and reaction 2 (B s R) being faster than reaction 1(B f P) since the chains are now close enough to easily entangle. The result is a n apparent two-step parallel mechanism.21 The low-density (26 &/molecule) 5,7-DCDA films behave very similarly to the low-density 10,12-PCDA(26 AVmolecule). Thus, the same mechanistic model and microscopic interpretation is relevant. The blue form of the polymer undergoes a reversible transition to the intermediate purple form which then rearranges to the red form a t sufficiently high temperatures. The reversible blue to red transition is not observed since the side chains are not close enough to allow good van der Waals interactions as discussed above. For the high-density films of the 5,7-diacetylenes, polymerization proceeded rapidly to the red form and there was little evidence of the blue form polymer even for short polymerization times. This is consistent with the side chains of the high-density 5,7 monomer films adopting a n arrangement that is nearly perpendicular to the diacetylene backbone so that the polymerization proceeds directly to the monoclinic form. This same behavior is observed for monomer films of 10,12-diacetylenes that have been annealed to greater than 60 “C prior to polymerization by exposure to W radiation.38 This behavior is shown by small-angle x-ray scattering to be due to a rearrangement in the monomer multilayers from a n orthorhombic arrangement of side chains to a triclinic arrangement of side chains.38 When the high-density 5,7-polydiacetylene films are heated, a slight shift of the visible spectrum toward the red form spectrum results. However, little or no reversibility is observed for these films. This suggests that reaction 1 (B P) and reaction 3 (P R) occur more quickly than any R B transition can occur. This is consistent with the fact that the closely packed side chains are easily tangled irreversibly during heating or during polymerization. The fact that the “red” spectrum is less defined for the high-density films than for the low-density films is consistent with a less ordered red form polymer a t higher densities.lg
-
-
-
Acknowledgment. A.A.D. acknowledges the Clare Boothe Luce Fund for their gracious support in the form of a Clare Boothe Luce fellowship. Acknowledgment is also made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. The purchase of the Perkin Elmer Lambda 4c UV-visible spectrometer was made possible by the gracious support of the NSF under Grant CHE-8611588. In addition, the authors thank The College of the Holy Cross for the support of this research. LA9406259
