Langmuir 1994,10, 1948-1954
1948
Kinetics of the Reversible Thermochromism in Langmuir-Blodgett Films of Cd2+Salts of Polydiacetylenes Studied Using UV-Vis Spectroscopy Alice A. Deckert,’ Lara Fallon,+Lisa Kiernan,t Christopher Cashin, Anthony Perrone, and Terry Encalarde Department of Chemistry, College of the Holy Cross, Worcester, Massachusettes 01610 Received September 27,1993. I n Final Form: February 28,1994@ The first quantitative model for the partially reversible thermochromism in Langmuir-Blodgett films of the polymerized Cd*+salts of lO,l2-tricosadiynoicacid (TCDA)and 10,12-pentacosadiynoicacid (PCDA) is presented. The visible spectrumas a function of temperature provides evidence for two parallel processes, one of which is reversible. The following kinetic model is proposed which qualitatively and quantitatively accounts for the observed reversible thermochromism: B F? R (kf,k,); P R (kd. B and P stand for two distinct forms of the blue polymer, and R stands for the red form of the polymer. Activation barriers of Ef = 22.5 kcal/mol, E, = 21.4 kcal/mol, and E2 = 23.0 kcal/mol are obtained from the TCDA spectra as a function of temperature using a “normal”preexponential factor of 1012 5-1 and Ef and E, as adjustable parameters. The same model can be fit to f i i of PCDA and gives activation barriers of Ef = 21.5 kcal/mol, E, = 21.0 kcal/mol, and E2 = 22.5 kcal/mol.
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Introduction Langmuir-Blodgett (LB) films provide a reproducible and experimentallysimple method for producing organized media. Indeed LB films have been utilized for everything from model membraned to dielectric spacers in studies of energy t r a n ~ f e r . ~These a 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 literature.”’ Unfortunately, LB films have been underutilized for kinetic or mechanistic studies of reactions in organized media.12J7-20
* T o whom correspondence should be addressed. + Current address: Department of Chemistry, Duke University, Durham, NC 27706. t Current address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. *Abstract published in Advance ACS Abstracts, May 1, 1994. (1) Nikolelis, D.; Brennan, J.; Brown, R.; McGibbon, G.;Kroll, U. Analyst 1991, 116, 122. (2) Arden.. W.:. Fromherz. P. Ber. Bunsen-Gee. Phys. Chem. 1978,82, 868. ’ (3) Yamazaki, 1.; Tamai, N.; Yamazaki, T.; Murakami, A.; Mimuro, M.; Fujita, Y. J. Phys. Chem. 1988,92,5035. (4) Barraud, A.;Rosilio,C.;Ruaudel-Teixier, A. ThinSolidFilms 1980, 68, 7. (5) Bubeck, C. Thin Solid F i l m 1988, 160, 1. (6) Ruaudel-Teixier. A.: Le1ouD.J.; Barraud, A. Mol. Cryst. Liq. . Cryst. 1986,134, 347. (7) Whitten.G.:Eaker.D.W.:Horsev.B.E.:Schmehl.R.H.:Worsham. , . P. R.‘Ber. Bunsen-Ges. Phys. Chem. i978,82,858. (8) Banerjie, 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. (16) Rabe, J. P.: Rabolt, J. F.; Brown, C. A.: Swalen. J. D. Thin Solid F i l m 1985,133, 153. (17) Laxhuber, L. A.; Scheunemann, U.; Mohwald. H. Chem. Phvs. 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.
The inherently low numbers of molecules in an LB film, coupled with the low sensitivity of most techniques used to obtain species concentrations, lead 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 suprising. The photopolymerization and subsequent thermochromism in LB films of the various diacetylenes of the form RC*C=C(CHz),COOH are well documented.l74 These diacetylenes undergo rapid polymerization when exposed to UV light (-254 nm). The polydiacetylene formed has a rigid, one-dimensional,conjugated backbone. This conjugation results in a strong ?r to ?r* 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. (20) Koshihara, S.;Tokura, Y.; Takeda, K.; Koda, T.; Kobayashi, A. J. Chem. Phys. 1990,92,7581. (21) Tieke, B.; Bloor, D. Makromol. Chem. 1979,180, 2275. (22)Kaneko, F.; Dresselhaus, M. S.; Rubner, M. F.; Shibata, M.; Kobayashi, S. Thin Solid F i l m 1988,160, 327. (23) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. (24) Kajzar, F.; Messier, J. Thin Solid F i l m 1983, 99, 109. (25) Tamura, H.; Mino, N.; Ogawa, K. Thin Solid F i l m 1989,179,33. (26) Fischetti, R. F.; Filipkowski, M.; Gerito, A. F.; Blasie, J. K. Phys. Rev. B 1988,37,4714. (27) Miyano, K.; Maeda, T. Phys. Rev. B 1986,33,4386. (28)Nakahara, H.; Fukuda, K.; Seki, K.; Asada, S.;Inokuchi, H. Chem. Phys. 1987,118,123. (29) Lyall, I. R. J.; Batchelder, D. Br. Polym. J. 1985, 17, 372. (30) Eckhardt, H.; Baudreaux, D. S.; Chance, R. R. J . Chem. Phys. 1986,85,4116. (31) Chollet, P. A.; Kajzar, F.; Messier, J. Thin Solid F i l m 1985,132, 1.
R.;Prasad, (32) Berkovic,G.;Superfhe,R.;Guyot-Sionnest,P.;Shen,Y. P. N. J. Opt. SOC. Am. B 1988,5, 668. (33) Sauteret, C.; Hermann, J. P.; Frey, R.; Pradere, F.; Ducuing, J.; Baughman, R. H.; Chance, R. R. Phys. Reu. Lett. 1976,36,956. (34) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980,68,77. (35) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci. 1979, 17, 1631. (36)Bloor, D.; Ando, D. J.; Obhi, J. S. Makromol. Chem., Rapid Commun. 1986, 7,665. (37) Okada, S.;Nakanishi, H.; Matsuda, H.; Kato, M.; Sugi, M.; Saito, M.; Iizima, S. Polym. J . 1988, 20, 493. (38)Saito, K.; Saito, M.; Ikegami, K.; Kuroda, S.; Sugi, M. Jpn. J. Appl. Phys. 1988,27, 1038.
0743-7463/94/2410-1948$04.50/00 1994 American Chemical Society
Reversible Thermochromism in LB Films
A wide variety of diacetylenes with the general formula RC=CC=C( CH2),COOH have been studied in LB films. All of these molecules undergo rapid photochemical polymerizationwhen exposedto UV light at approximately 254 nm.1738 This polymerization is apparent in both LB films and in the solid state as a color change.l7-S 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.15 This chromism can also be initiated by solvents such as chloroform or ethan01.l~ Although some controversy still exists in the literature,6J5J8J9the most widely accepted explanation for the thermochromism is a phase change from a monoclinic to an orthorhombic arrangement of the side chains R and (CH2),COOH.26 This rearrangement of the side chains causes a stress to the conjugated backbone of the polymer, resulting in a shift of the r a n d r* electronic energylevels.30 The thermochromic change in polydiacetyleneshas been shown to be reversible.la20 The thermochromism was completely reversible up to temperatures below the polymer melt temperature.l8v2O Once the polymer had melted and the interunit hydrogen bonding was thus interrupted, or the polymer side chains R and (CH2),COOH became entangled, the thermochromic change was irreversible.18920The current investigation provides the first quantitative kinetic and mechanistic study of the thermochromism in Cd2+salts of 10,12-tricosadiynoicacid (TCDA) and 10,12-pentacosadiynoic acid (PCDA).
Langmuir, Vol. 10, No. 6, 1994 1949 60
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Area per Molecule (x) Figure 1. Typical pressure vs area isotherms of (a) 10,12tricosadiynoicacid and (b) 10,12-pentacosadiynoicacid on a water subphase at T = 288 K, with [Cd2+l= 2 X 1o-L M and pH 6.67. The arrow indicates the pressure of 20 dyn/cm at which fiis were transferred for kinetic analysis. This pressure corresponded to a molecular area of approximately 23 A2/moleculefor both TCDA and PCDA.
hydrophobic by placing them in a breaker containing 3 drops/ slide of 1,1,1,3,3,3-hexamethyldisilizane.The beaker was convered with a watch glass and let stand overnight. Polymerization of the 30-layer TCDA and PCDA 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 measuredusing a ferrioxalate chemicalactinomete~~l The spectral output of the Hg lamp was peaked at 254 nm and was approximately 99% 254-nm light. At a distance of 10 cm directly in front of the lamp, the photon flux was determined to be 3 X 1017 photons/(cm28 ) . Assuming only 254-nm light, this photon flux correspondsto a power density of 230 mW/cm2.Both Experimental Section sides of the glass substrates containing the 30-layer TCDA and PCDA LB films were exposed to the UV radiation from the Hg The preparation of the LB multilayers was accomplished using the well-documented Langmuir-Blodgett dipping meth0d.3~~~ lamp for various times to determine the time necessary to accomplishcompleteconversionto blue polymer without inducing The TCDA and PCDA, purchased from Farchan, were recrystalany conversionto red polymer." A time of 7 min/side was found lized from petroleum ether (bp W110 "C) immediately prior to to be satisfactory for TCDA. A polymerization time of 5 min/ volumetric preparation of an approximately 1 mg/mL solution side was used for PCDA films. in spectral grade chloroform. A microsyringewas used to spread The polymerization and subsequent thermochromism were 150 pL of solution dropwise over the subphase surface. The followed by optical absorption using a Perkin-Elmer lambda 4c subphase consisted of ultrapure water from a Millipore reverse UV-vis spectrometer in transmission mode. Thereference beam osmosis still with deionizing filters. The water had a resistivity of the double-beaminstrument was transmitted through air. The greater than 17 MQcm. Cadmium chloride was used to prepare repetitivescan feature of the spectrometer was utilized to obtain an approximately 1X 1o-L M solution of Cd2+in ultrapure water. spectra every 2 min as the temperature of the film was changed. This solution was brought to a pH of approximately 6.5 using Temperature control of the film was accomplished with a 0.01 M NaHC03. The subphase in the NIMA LB system was custom-made slide holder through which water from a thermoregulated at 15 OC. The entire LB trough system was enclosed stated bathwas circulated. Figure 2 showsthe slide holder which in a laminar flow hood providing class 100 clean air. After mounted directly in the sample beam path of the UV-vis spreading the TCDA and PCDA monolayer, the Langmuir film spectrometer. The temperature of the film was monitored using was compressed at a rate of 20 cm2/minto a surface pressure of a type K chromel-constantan thermocoupleattached with epoxy 20 dyn/cm. to the slide holder as shown in Figure 2. Slide temperatures Figure 1shows typical pressure-area isotherms of TCDA and could be read to h0.5 K with this arrangement. The water bath PCDA obtained for the above conditions. A pressure of 20 dyn/ could be cooledto 276 K and heated to 368 K. This temperature cm gave a molecular area of about 24 &/molecular for TCDA range allowed slide temperatures between 283 and 358 K. A and 23 AZ/molecule for PCDA, in agreement with the literanearly linear heating rate at the sample was achievedby applying ture.ls*% A compression speed of 20 cm2/mingave films with a constant current to the bath heater coil. A constant cooling stabilities that allowed for good transfer at 20 dyn/cm. rate was obtained by disablingthe water bath heater and allowing A constant surface pressure of 20 dyn/cm was maintained the refrigeration unit to cool at maximum. Slide temperatures throughout deposition of the multilayers. Thirty Y-type layers constant to *0.5 K could be maintained for up to 1 h. were deposited by passing the hydrophobic glass substrate through the air-layer-water or water-layer-air interface at a Results constant rate of 8 mm/min to produce one monomolecularlayer for each pass through the interface. Transfer ratios between Photopolymerization. Figure 3 shows the increase in 80% and 100% were obtained for both TCDA and PCDA films. the area under the visible absorption spectra between 450 The substrate consisted of a glass microscopeslide which had and 700 nm as a function of time exposed to the output been thoroughly cleaned with soap and water and subsequently of a Hg arc lamp. The 30-layer TCDA and PCDA films sonicated for 15min each in spectral grade chloroform,isopropyl were held at a low temperature of 284 K during exposure alcohol,subphase quality water, isopropylalcohol,and subphase in order to avoid thermal conversion to the red form of the quality water in that order. The slides were then rendered polymer film. Both sides of the glass slide were treated identically to ensure polymerization of the films on both (39)Blodgett, K.B.; Langmuir, I. Phys. Reo. 1937,51,964. (40)Gaines, G. L.Insoluble Monolayers at Liquid-Gas Interfaces; (41)Hatchard; Parker. R o c . R. SOC.London, A 1956,235,518. Interscience Publishers: New York, 1966.
1950 Langmuir, Vol. 10, No.6, 1994
Deckert et al. a
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Figure 2. An exploded view of the custom-designedcell used
for obtainingvariable-temperature transmission UV-vis spectra of the Langmuir-Blodgett films of TCDA on glass substrates. Film temperatures between 283 and 358 K could be achieved usingthis cell. Temperatureswere read using a type-K chromelconstantan thermocoupleattached to the cell as indicated in the figure.
280
330 Temperature (K)
380
Figure 4. (a) Visible spectrum of a 30-layer TCDA film polymerized for 420 s as a function of film temperature. Absorbancesare normalized to the peak absorbance at 630 nm. No isosbestic point is evident. (b) Normalized absorbances at 500, 540, 580, and 630 nm as a function of temperature. For decreasing absorbances with temperature, the normalization function ( A - A m ) / ( A o - A m ) is used. For increasing absorbances with temperature, the normalization function ( A- Ao)/(A, - Ao) is used. Two inflection points are observed. 1.2
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200
300
400
500
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Figure 3. Absorbance of the visible spectrum of (m) TCDA
and ( 0 )PCDA integrated between 700 and 450 nm for various polymerization times. For each time, t, both sides of the glass substrate were exposed to 254-nm light at a flux of 3 X l O I 7 photons/(cm2s). The photon flux from the Hg arc lamp was determined using a ferrioxilate actinometer. sides. Thus, a time of 100s indicatesthat both sides were exposed at a distance of 10 cm directly in front of the Hg lamp for 100 s. The kinetics of the thermochromism of TCDA was studied in films which had been polymerized for 420 s. The kinetics of the thermochromism in PCDA was studied in films which had been polymerized for 300 s. These polymerization times result in values for the integrated absorbance which are close to the maximum values attained. TemperatureDependence of Absorption Spectra. As seen in Figure 4, the thermochromism in the TCDA films can be easily followed using visible spectroscopy. The data in Figure 4a show the effect on the visible absorption spectra of a 30-layer TCDA film polymerized for 420 s as the temperature of the film is increased at a nearly linear heating rate of 0.04 K/s. Figure 5 shows analogous data for a 30-layer PCDA film polymerized for 300 s. The data in Figure 5 are obtained at a heating rate of 0.03 K/s. There are several notable features in these data. First,the spectraas a functionof temperature shown in Figures 4a and 5a do not exhibit an isosbestic point. The lack of an isosbestic point rules out any simpleprocess involving only two absorbing species, such as the blue and red forms of the polymer. A more complex mechanism involving at least three absorbing species must be postulated. The lack of an isosbestic point is corroborated in Figures 4b and 5b which show the normalized absorbance a t each
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.
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Figure 5. (a) Visible spectrum of a 30-layer PCDA film polymerized for 300 s as a function of film temperature. Absorbancesare normalized to the peak absorbance at 630 nm. No isosbestic point is evident. (b) Normalized absorbances at 500, 540, 580, and 630 nm as a function of temperature. For decreasing absorbances with temperature, the normalization function ( A - Am)/(Ao- A m ) is used. For increasingabsorbances with temperature, the normalization function ( A - A o ) / ( A m - Ao) is used.
of the maxima of the absorption spectra as a function of temperature. For the wavelengths associatedwith the blue polymer (630 and 580 nm) which give decreasing absorbances with temperature, the absorbances are normalized according to ( A - A,)/(Ao - A,). For wavelengths associated with the red form of the polymer (540 and 500 nm) which give increasingabsorbances with temperature, the function (A - Ao)/(A, - Ao) is used to normalize the absorbances. The information obtained from each wavelength in the visible absorption spectra is distinctly different. Figure 4b clearly shows two inflection points in the normalized absorbance a t 580 and 500 nm as a function of temperature. The data at 630 and 540 nm show these two inflection points as well; however, the second break is less pronounced. Although the data for PCDA shown in Figure 5b also reveal two inflection points, they are not as pronounced as those observed in Figure 4b. The normalized absorbances in Figures 4b and 5b are proportional to the two-dimensional concentration, or coverage, of the species in the film, which absorb at the wavelength of interest. Thus, an inflection point in Figure 4b (or Figure 5b) represents a point where the second
Reversible Thermochromism in LB Films
Langmuir, Vol. 10, NO.6, 1994 1951
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Figure 6. (a) Visible spectra of a 30-layer film of polymerized TCDA as a functionof temperature. Absorbancesare normalized to the peak at 630 nm. The change in the spectrum observed upon heating the f i b to 320 K is completelyreversed upon cooling the film back to 287 K. (b) Normalized absorbances at 630,580, 540, and 500 nm as a function of temperature. The reversibility of the change from the blue form of the polymer (630 and 580 nm) to the red form of the polymer (540and 500 nm) as a function of temperature is evident.
derivative of the coverage with respect to temperature goes to 0. Since the rate of reaction can be expressed as the first derivative of the coverage with respect to temperature multiplied by 1 over the heating rate, the temperature at the inflection point represents that temperature at which the rate of reaction is at a maximum. The existence of two inflection points in the absorbance data as a function of temperature is consistent with two kinetic processes occurring with two distinct rates. This suggests two types of polymer in the film which behave differently. The existence of two forms of the blue polymer is also consistent with the lack of an isosbestic point in the spectra as a function of temperature and has been observed previously.l8I2l If two blue forms exist, then at least three must be present species, blue (B),"purple" (P),and red (R), in the film as the conversion from blue to red occurs. Given the temperatures a t the inflection points of 326 and 350 K for TCDA, and assuming first-order Arrhenius rate constants, an estimation of the activation barriers for the two processes resulting in 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[Bl, where [B] stands for the coverage of blue polymer in the film, and k = Ae(-E/*n is the first-order Arrhenius rate constant. The inflection point occurs when dr/dT = 0 = k(d[Bl/dn + (dk/d7')[Bl. Differentiating, rearranging, and recognizing that dTldt = @ = heating rate lead to
The activation barriers a t which these two functions are equal for the temperature of the inflection points are calculated assuming a "normal" preexponential for a firstorder process of 1012s-l. This calculation gives activation barriers of 21.4 and 23.0 kcal/mol for the TCDA data presented in Figure 4b. A similar calculation gives activation barriers of 21.2 and 22.5 kcal/mol for the PCDA data shown in Figure 5b. Thermal Cycles. In order to observe any reversible thermochromism, thermal cycling experiments are performed. Figure 6a shows a typical set of absorption spectra resulting from a thermal cycle in which the temperature of the 30-layer TCDA polymer filmis increased a t a heating
630 nm
560 540
500
b 343 c 287 550
650
Wavelength (nm)
280
320 360 Temperature (K)
Figure 7. (a) Spectra of a 30-layer film of polymerized TCDA as a function of temperature. Absorbances are normalized to the peak at 630 nm. Heating the film to 343 K resulted in nearly complete conversion of the blue form of the polymer (630 and 580 nm) to the red form (540 and 500 nm). Cooling of the film to 286 K resulted in partial reversibility back to the blue form of the polymer. (b) Normalized absorbances at 500, 540, 580, and 630 nm as a function of temperature. An inflection point at 326 K is evident. The partial reversibility of the change from the blue form of the polymer to the red form of the polymer is
observed.
rate of 0.04 K/s from 284 K to just before the first inflection point, 320 K. The film temperature is then decreased a t arate of 0.02 K/s from 320 to 285 K. A significant decrease in the absorbance due to the blue form polymer at 630 and 580 nm and an increase in the absorbance due to the red form polymer at 540 and 500 nm occur by the time the film has reached 320 K. Cooling of the film to 285 K results in nearly complete recovery of the peaks at 630 and 580 nm attributed to the blue form polymer. These data are shown again in Figure 6b as the normalized absorbance as a function of temperature. Figure 6b clearly demonstrates the reversibility of the thermochromism between 285 and 320 K. Similar results were obtained for 30-layer PCDA films annealed between 287 and 317 K. Figure 7 shows typical results obtained when the same TCDA film is subjected to a thermal cycle between 285 and 343 K. The temperature of 343 K is past the first inflection point, but prior to the second inflection point observed in Figure 4b. As seen in Figure 7a, increasing the temperature to 343 K results in nearly complete conversion of the spectra associated with the blue polymer to the spectra associated with the red polymer. Cooling the film to 284 K results in only about 20% recovery of the absorbance in the blue polymer region a t 630 nm and about 60% recovery of the absorbance at 580 nm. When the PCDA film is annealed to 329 K and back to 287 K, a 73% recovery of the absorbance in the blue polymer region at 630 nm and a 65% recovery at 580 nm are observed. The partial, but not complete, reversibility of the thermochromism between 285 and 343 K for TCDA and 287 and 329 K for PCDA suggests that only one of the processes observed in Figures 4 and 5 is reversible. Thus, one process acts as a sink for the red form polymer, resulting in incomplete reversibility of the absorption spectra as a function of temperature. If the temperature of the same TCDA film is subsequently raised to 358 K, resulting in complete conversion to the red form polymer, and then decreased again to 285 K, only partial reversibility is again observed. These data are displayed in Figure 8. Note that the initial spectrum at 287 K in Figure 8 corresponds exactly to the final spectrum at 287 K shown in Figure 7. Figure 8b shows a recovery of the absorbance at 630 nm of about 3 % and a
Deckert et al.
1952 b
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550 650 Wavelength (nm)
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330 Temperature (K)
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Figure 8. (a) Spectra of a 30-layer polymerized TCDA film as a function of temperature. Absorbances are normalized to the peak at 540 nm. Heating the film to 358 K resulted in complete conversion of the blue form of the polymer (630 and 580 nm) to the red form (540and 500 nm). Cooling the film to 287 K resulted in only slight reversibility back to the blue form of the polymer. (b) Normalized absorbances at 500, 540, 580, and 630 nm as a function of temperature. An inflectionpoint at 350 K is evident. The partial reversibility of the change from the blue form of the polymer to the red form of the polymer is observed. 1 ,
--
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300
320 Temperature (K)
340
360
Figure 10. Normalized absorbances of a PCDA film at 500 and 630 + 580 nm as a function of temperature. The decrease in the sum of the absorbances at 630 and 580 nm fully accountsfor the increase in the absorbance at 500 nm. The solid lines represent a two-parameterfit to the data using the kinetic model discussed in the text. of two forms of blue polymer suggested by the lack of an isosbestic point in the visible absorption spectra as a function of temperature. Proposed Model. The following kinetic model for the thermochromism is consistent with all the data presented:
I
Blue 0
Red
-
0 280
300
320 Temperature (K)
360
340
Figure 9. Normalized absorbances of a TCDA film at 500 and 630 + 580 nm as a function of temperature. The decrease in the sum of the absorbances at 630 and 580 nm fully accounts for the increase in the absorbance at 500 nm. The solid lines represent a two-parameterfit to the data using the kinetic model discussed in the text. recovery of the absorbance a t 580 nm of about 41 7%. The peaks at 540 and 500 nm attributable to the red form polymer both decrease by approximately 17% Annealing the PCDA film from 287 to 356 K and cooling back to 287 K resulted in a recovery of the absorbance at 630 nm of about 7 % and recovery at 580 nm of 30 96,and the peaks at 540 and 500 nm attributable to the red form of the PCDA polymer both decrease by about 10%. Figures 6-8 clearly show that the absorbance at 630 and 580 nm, attributable to the blue form of the TCDA polymer, behaved differently as a function of temperature. However, the absorbances at 540 and 500 nm, attributed to the red form of the TCDA polymer, are not nearly so different. The data for the PCDA films were analogous. To obtain the data shown in Figures 9 and 10, the absorbances at 630 and 580 nm were added together and the result normalized and plotted as a function of temperature. These data are shown together with the normalized absorbance at 500 nm. Addition of the absorbances at 630 and 580 nm results in the mirror image of the data at 500 nm. That is, the disappearance of the combination of the 630- and 580-nm absorbances exactly accounts for the appearance of the absorbance a t 500 nm. This suggests that the appearance of the red form polymer can only be accounted for completely by the disappearance of more than one species with an absorbance between 580 and 700 nm. This is again consistent with the presence
.
The symbols B and P represent the forms of the blue polymer, and R represents the red polymer. This model accounts qualitatively for the partial reversibility of the thermochromism, the lack of an isosbestic point, and the evidence for two processes resulting in loss of absorbance in the blue polymer region and increase in absorbance in the red polymer region of the visible absorption spectra. In addition, the decrease as a function of temperature in B and P represented by the normalized sum of the absorbance at 630 and 580 nm and the increase in R as a function of temperature represented by the normalized absorbance a t 500 nm for both TCDA and PCDA films is quantitatively predicted by this model. Numerical integration of the rate equations using Efand E, as adjustable parameters resulted in the solid lines shown in Figures 8 (TCDA) and 9 (PCDA). The reversible process was assignedto the first inflection point which is at 326 K for TCDA and 327 K for PCDA. Best fits to the model reveal that assigning the second inflection point to the reversible process results in a fit to the data that cannot account for any recovery of the absorbances at 630 and 500 nm during the cooling cycle. Only fits in which the reversible process corresponds to the first inflection point account quantitatively for the observed 17% recovery of the absorbances for TCDA and the 10% recovery of the absorbances for PCDA. In addition, assigning the first inflection point to the reversible process results in AH > 0 in agreement with previous work.18s20 Best fits in which the second inflection point is assigned to the reversible process result in AH < 0. The fraction of red polymer present a t the start of the temperature cycle, Ro, is assumed to be zero. Thus, the fraction of Bo POis equal to unity. For the TCDA data, the process corresponding to the first inflection point results in depletion of approximately0.83(& +PO).Thus, the form of the blue polymer (B)responsible for the observation of the inflection point at 326 K is assigned an
+
Langmuir, Vol. 10, No. 6, 1994 1953
Reversible Thermochromism in LB Films
blue
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*
6dC' red
Figure 11. Schematic diagram of the kinetic model discussed in the text. The polymer backbone is depicted as a straight line
to emphasize the interactions in the side chains. The cadmium salt of the carboxylate anion is depicted as a filled circle. The activation barriers displayed on the reaction arrows are in units of kilocalories per mole. Only the activation barrier for TCDA deduced from the best fit to the temperature cycling data is displayed. initial fraction of Bo = 0.83. The remaining fraction of 0.17 is assigned to PO. The activation barriers for the irreversible process are obtained from the analysis of the temperature at the inflection points described earlier. Sincethe first inflection point is due to a reversible process, a simple analysis using the temperature at the inflection point and assuming firstorder kinetics is not appropriate. Thus, the activation barriers for the forward and reverse processes are used as adjustable parameters to obtain the best fit to the data at 500 nm. The model is fit to the data at 500 nm since these data are least affected by any absorbance from the two types of blue polymer. Although the simple sum of the data at 630 and 580 nm seems to account for the change in absorbance at 500 nm, this sum cannot be assumed to quantitatively represent the loss of B P since the molar absorptivities of these species are not known. The twoparameter fit shown in Figures 9 and 10 as solid lines uses the following values of all parameters: (TCDA) Ro = 0, Bo = 0.83, Ef = 22.5 kcal/mol, E2 = 23.0 kcal/mol, E, = 21.4 kcal/mol, Af = A, = A2 = 10l2s-l; (PCDA) RO = 0, Bo = 0.45, Ef = 21.5 kcal/mol, E2 = 22.5 kcal/mol, E, = 21.0 kcal/mol, Af = A, = A2 = 1012 s-1. The model using the above parameters also predicts the observed 17 % recovery of the 580 + 630 nm absorbance and the observed 17% decrease in the 500-nm signal when the temperature of the TCDA films is decreased from 358 back to 284 K. Likewise the 105% recoveryof the 580 630 nm absorbance and the observed 10 % decrease in the 500-nm signal when the temperature of the PCDA films is decreased from 356 to 287 K are predicted by the model using the parameters above. Figure 11shows the deduced reaction scheme with the experimentally determined activation barriers for TCDA.
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Conclusions A quantitative kinetic model for the thermochromism in LB films of TCDA and PCDA is presented which is consistent with all data obtained. The simplest possible model to account for all the data involves two different types of blue polymer. These two types of polymer undergo a thermochromic change to the red form polymer each at a distinct rate. One of these rate processes is reversible; the other is irreversible and acts as a sink for the red form TCDA polymer. The present study does not provide any direct evidence for the identity of the B and P species. The existence of ordered and disordered domains in the LB film seems
likely, and is documented for films made under similar condition^.^^-^^ The hydrocarbon side chains of the conjugated backbone in the ordered domains of the polymer film are in an all-trans configuration. Disordered domains contain various degrees of gauche interactions, shown schematically in Figure 11,within the hydrocarbon side chains.18126~34 The hypothesis that ordered and disordered domains undergo the thermochromic change to red form polymer at different rates is particularly compellingif the thermochromism is due to a phase change from an orthorhombic structureto a monoclinic structure.26 This phase change is accomplished by a tilt of the hydrocarbon side chains depicted schematically in Figure 11.26The larger number of gauche interactions within the disordered domains would be expected to increase the ground-state internal energy of the polymer film. In addition, the transition state in this process would be expected to be a high-energy state. As the chains begin to tilt, the gauche interactions destabilize the transition state by forcing methylene groups to interact too closely with neighboring side chains. On the other hand, ordered domains with the side chains in an all-trans configuration would be expected to undergo the process through a lower energy transition state since the all-trans configuration is easily maintained throughout the tilting process, stabilizing the transition state. Although both the ground state and the transition state are destabilized by gauche interactions, the data suggest that the transition state is destabilized to a greater extent. This picture is also supported by similar studies performed on poly-U4n LB films.19p20 These diacetylenes contain urethane groups in the side chains, and thus LB films of poly-U4n have the same conjugated backbone with the added possibility of strong interunit hydrogen bonds between urethane groups.20 A reversible thermochromism is observed for these LB films. However, beyond the polymer melt temperature, the thermochromic change back to the blue form polymer can no longer be observed.20 After the polymer has undergone a melting transition, hydrogen bonding between urethane groups is nearly completely disrupted.20 In the poly-4Un films AH'S of 1-3 kcal/mol were observed depending on the length (n) of the hydrocarbon side chains. This is consistent with the AH'S of 1.1and 0.5 kcal/mol observed for TCDA and PCDA, respectively. The picture of ordered and disordered blue forms of the polymer, suggest the existence of both ordered and disordered red forms of the polymer. Given the activation barriers obtained from the data, the kinetic scheme shown in Figure 11can be deduced. In this diagram, the reverse reaction R P would have an undetermined activation barrier. Although the transition state for R B is attainable at the temperatures investigated,the transition state for R P need not be accessible even if only one red form of the polymer exists. Thus, the data do not require that a second (disordered) red form of the polymer exist, but neither do the data rule out such a possibility. The simplest mathematical model, involving only one red form of the polymer, fits the data well. Further experiments are needed to determine if two forms of the red polymer do in fact exist. An alternative kinetic model has been proposed to account for the partial reversibility of the thermochromism in PCDA.18 This model involves two serial processes in which one is reversible. This model can account for the lack of an isosbestic point in the spectra as a function of temperature and the partial reversibility of the absorbances due to the blue form of the polymer. However,
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1954 Langmuir, Vol. 10, No. 6, 1994 this model cannot account for any decrease in the absorbances due to the red form of the polymer upon cooling the film. The data presented here for TCDA and PCDA clearly show a reversibility in the absorbance at 500 nm which is attributed to the red form of the polymer. In conclusion, a quantitative model for the thermochromism in polydiacetyleneLB films has been proposeod. The existence of both ordered and disordered forms of the blue polymer are suggested by the data. The thermochromism is a result of a phase transition from an orthorhombic to a monoclinic structure and takes place at different rates dependingon whether the film is ordered
Deckert et al. or disordered. The proposed kinetic model supports this picture. Acknowledgment. A.A.D. wishes to acknowledgethe 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 partial support of this research. The purchase of the Perkin-Elmer Lambda 4c UV-vis spectrometer was made possible by the gracious support of the NSF under Grant No. CHE-8611588. In addition, the authors wish to thank the College of the Holy Cross for support of this research.
