J. Phys. Chem. 1986, 90, 3031-3033
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Temperature Dependence of Electronic Absorption in Poiy(diacety1ene) Solutions R. R. Chance,* J. M. Sowa, H. Eckhardt, Corporate Research and Technology, Allied-Signal Corporation, Morristown, New Jersey 07960
and M. Schott Groupe de Physique des Solides de I’Ecole Normale SupZrieure, Universite Paris 7, Paris, France (Received: December 17, 1985)
Poly3BCMU and poly4BCMU, poly(diacety1ene)s with bulky urethane substituents, readily dissolve in common organic solvents such as chloroform to yield yellow polymer solutions. On addition of a nonsolvent, such as hexane, dramatic color transitions take place (yellow to blue for poly3BCMU and yellow to red for poly4BCMU). These color changes are indicative of conformation transitions involving ordering of the polymer chains in solution. In this paper, we report the temperature dependence of optical absorption spectra for these polymers in various mixtures of chloroform and hexane. We observe a temperature-induced color transition, analogous to the solvent-induced one, in poly4BCMU but not for poly3BCMU. We also report the first observation of a “red phase” for poly3BCMU solutions at low temperature. These results point to fundamental differences in the conformation transitions of these two polymers in solution.
Introduction High molecular weight poly(diacety1ene)s are generally insoluble even in exotic organic solvents. The first exceptions to this rule were the butoxycarbonylmethyleneurethane (BCMU) substituted poly(diacety1ene)s.I The substituents for these polymers, commonly referred to as poly3BCMU and poly4BCMU, are shown as follows: 0 0
3BCMU
II
II
-(CH2)3-O-C-N-CH*-C-O-C4HS I
H 0
4BCMU
II
0 II
-(CH2,,-O-C-N-CH,-C-O-C4H~ I H These poly(diacety1ene)s (PDA) are soluble in common organic solvents, such as CHC13, due to the high entropy content of the complicated urethane substituent groups. The synthesis of these soluble polymers has led to a number of interesting developm e n t ~ , ’ -perhaps ~ the most remarkable being the discovery of a conformational transition in the polymer solutions, referred to as a “visual conformational transition”.’ The conformational transition in BCMU polymers is induced by a change in solvent or temperature. Though conformational transitions are commonly observed in polymer solutions, the PDA system is unique because of the sensitivity of the electronic properties to backbone conformation, which results in dramatic color changes during the conformational transition. This sensitivity to conformation originates from the conjugated structure of the PDA backbone [=RC-C=C-CR=]. *-Electron conjugation requires a planar structure. Conformational disorder limits conjugation by disrupting planarity to produce a distribution of chromophores of various “conjugation lengths” on the individual chains. The electronic properties vary with conjugation length, usually with a length-’ d e p e n d e n ~ e . ~Short conjugated PDA segments absorb in the blue spectral region (yellow solutions) while long segments absorb in the red (blue solutions). Therefore, the conformation transition in soluble PDAs is thought to involve a significant change in conjugation length. In poly4BCMU, the (1) Patel, G. N.; Chance, R.; Witt, J. J . Chem. Phys. 1979, 70, 4387. (2) Patel, G. N.; Walsh, E. J . Polym. Sei. (Lett.) 1979, 17, 203. (3) Lim,K. C.; Heeger, A. J. J . Chem. Phys. 1985,82, 522. (4) Chance, R. R.; Shand, M. L.; Hogg, C.; Silbey, R. Phys. Rev. B 1980, 22, 3540.
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transition is accompanied by a yellow-to-red color change, a 2000-cm-’ shift in optical absorption. In poly3BCMU the color change is yellow to blue, an absorption shift of more than 5000 cm-I. In the blue solution phase of poly3BCMU the optical properties are very similar to those of its crystalline form,s a result which suggests an effectively infinite conjugation length in the blue solutions. Additional soluble poly(diacety1ene)s have been discovered which show temperature- or solvent-induced color changes with absorption shifts similar to ~ o ~ ~ ~ B C MThese U . ~solutions * are believed to consist of aggregates in the red p h a ~ e . ~Evidence .~ is mixed as to whether polyBCMU solutions in their red or blue phases consist of aggregates or single chains. Lim and Heeger,3 using light-scattering measurements, argue for the single chain interpretation based on the lack of variation of the hydrodynamic radius with polymer concentration. Wenz et al.699 suggest the solutions are composed of aggregates as large as 700 chains. These light-scattering measurements on the solutions cannot address the important question of the molecular origin of the conformational transition, and in particular whether it is intra- or intermolecular. Previous spectroscopic’ and kineticlo work has suggested that the driving force for the transition is purely intramolecular in origin (involving the hydrogen-bonding network of the urethane substituent groups) and suggests that, if aggregation takes place, it must be subsequent to the conformational transition. Recent experiments involving field-induced birefringence in poly4BCMU solutions strongly support the intramolecular interpretation of the color transition.” In this article, we present data on the temperature dependence of UV-visible spectra for poly4BCMU and poly3BCMU solutions in various mixtures of CHC13 and hexane. The data suggest that there may be fundamental differences in the nature of the conformational transitions in these two polymers which have not been recognized in previous work. (5) Chance, R. R.; Patel, G.; Witt, J. J . Chem. Phys. 1979, 71, 206. (6) Wenz, G.; Wegner, G. Makromol. Chem. Rapid. commun. 1982, 3, 206. (7) Plachetta, C.; Rau, N . 0.;Hauck, A.; Schultz, R. C. Makromol. Chem. Rapid. Commun. 1982, 3, 249. (8) Plachetta, C.; Schultz, R. C. Makromol. Chem. 1982, 3, 815. (9) Wenz, G.; Muller, M.; Schmidt, M.; Wegner, G. Macromolecules 1984, 17, 837. (10) Chance, R. R.; Washabaugh, M. W.; Hupe, D. J. In Polydiacetylenes, Chance, R. R., Bloor, D., Ed.;Martinus Nijhoff: The Netherlands, 1985; p 239. Chemtronics 1986. 1. 36. (1 1) Lim, K. C.; Kapitulnik, A.; Zacher, R.; Heeger, A. J. J . Chem. Phys. 1985, 82, 516.
0 1986 American Chemical Society
3032 The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 700 600 I
I
WAVELENGTH (nm) 500 450
700 600
500 450 I
1
Xc=l.0
,
,
700 600
500 450
400
I
--
45--
22"
Chance et al.
I
'
24"
Xc = 0.55 A
I
I
X, = 0.47
I
t
1
23" 1 16 18 20 ,
I
I
1 , 22 24 1
Figure 1. Variation of optical absorption spectra with temperature ("C) for poly4BCMU in CHClJhexane at various mole fractions of CHClpin hexane (XCh
Experimental Section The monomers, 4BCMU and 3BCMU, were synthesized as described elsewhere' and polymerized via 50-Mrad60Co y-ray irradiation. Unreacted monomer was removed by extraction with acetone and the polymers were then dissolved in CHC13to produce yellow solutions. Two additional solutions were prepared for each polymer by the addition of hexane, a nonsolvent which induces the color transition. One solution had sufficient hexane to stabilize the red phase for poly4BCMU (or the blue phase for poly3BCMU), but not enough hexane to cause precipitation. The other solution had sufficient hexane to place the solvent composition as near as possible to the color transition point, but to still maintain a predominantly yellow solution. For poly4BCMU, the color transition point is approximately X , = 0.55, where X , is the mole fraction of CHCl, in hexane; the three solutions chosen for study had X , = 1.0,0.55, and 0.47. For poly3BCMU, the color transition point is approximately X , = 0.71; the three solutions studied had X , = 1.0,0.75, and 0.62. The polymer concentrations were in all cases 0.0002 mol/L, where mol refers to moles of monomer repeat units. UV-visible spectra were recorded with an apparatus described elsewhere.' The bandpass was 0.7 nm. A 0.1-cm pathlength quartz solution cell was in contact with a copper coldfinger equipped with a heating coil; the coldfinger was attached to a reservoir to which liquid nitrogen could be added in aliquots. The cell and coldfinger were contained in a chamber which was constantly purged with dry nitrogen to avoid water condensation. The lowest temperature achieveable with this arrangement was about -70 O C . Except for a small window for the probe beam, the cell had a thick coating of conducting silver paint for improved thermal conductivity. Thermocouples were placed immediately above and below the window. The temperature gradient across the window varied from a few degrees at 0 OC to about 8 deg at -70 O C ; averaged values are quoted herein. Results and Discussion Spectra of poly4BCMU solutions at various temperatures are shown in Figure 1. In this system, addition of hexane causes a yellow-to-red color change; the corresponding spectral changes
can be seen from the three room temperature spectra in the figure. Overall the spectral shift during the conformational transition is about 2000 cm-I. On lowering the temperature for the yellow ( X , = 1.O) solutions, there is a slight red shift in the spectra and some indication of additional ordering of the chains (as judged from the slight structure that develops in the spectra at low temperatures). These changes are completely reversible. The X , = 0.55 spectra show substantial changes with temperature, indicating a variation in polymer conformation. The changes are similar to, but even more dramatic than, the spectral variations with solvent in the CHC13/hexane system or those with temperature for poly4BCMU in t ~ l u e n e .The ~ spectrum obtained at -66 OC is remarkably sharp, indicating a well-ordered polymer chain. Yet there is no indication of precipitation and all the spectral variations are completely reversible with no observable hysteresis. For X , = 0.47 which yields the red phase at room temperature, there is little variation with temperature. Below -72 "C, the polymer precipitates; in fact aggregation of the chains may be occurring at higher temperatures. For this reason perhaps, the small spectral variations with temperature that are observed in Figure 1 for X , = 0.47 are not completely reversible. Lim and Heeger3 have studied the temperature dependence of poly4BCMU in CHC13/hexane in the region above room temperature. Their results are in basic agreement with ours-the key observation being that temperature can induce the color transition reversibly with no significant hysteresis. Results for poly3BCMU are shown in Figure 2. In this system, hexane causes a yellow-to-blue transition with an overall shift in optical absorption of about 5000 cm-I. The yellow solution ( X , = 1.0) is nearly identical with that of poly4BCMU, except slightly red-shifted. When this solution is cooled a more structured absorption band develops, which is nearly identical with the spectrum of the room temperature, red phase of poly4BCMU. This is the first observation of a "red phase" for poly3BCMU. At X , = 0.75 the 24 OC spectrum contains both yellow and blue components. As the temperature is lowered, there is no measurable change in the relative fractions of the blue and yellow components. We have also heated the X , = 0.75 solution to about 60 OC and observed some decrease in the blue component; this may be due to deg-
Electronic Absorption in Poly(diacety1ene) Solutions
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The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 3033
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Figure 2. Variation of optical absorption spectra with temperature (“C) for poly3BCMU in CHCI3/hexane at various mole fractions of CHCI, in hexane (XJ.
radation, however, since the effect is not reversible.I2 Note that the yellow component yields the “red phase” on cooling, the spectrum being nearly identical with that for X, = 1.0. The blue component also shows some additional structure on cooling. All of these spectral variations with temperature for both X , = 1.0 and X, = 0.75 are completely reversible with no observable hysteresis. We conclude that the yellow-to-blue color transition cannot be induced by temperature in the CHC13/hexane solvent system. For X, = 0.62, there is a distinct evolution in the spectra on cooling toward a more structured absorption band which is very similar to that of crystalline poly3BCMU. Below -24 OC, the polymer precipitates. Our results for poly3BCMU in CHCl,/hexane are in marked contrast to those for poly3BCMU in dichloroethane reported by Wenz et aL9 They report a temperature-induced, blue-to-yellow color transition a t about 60 O C . From their data, the transition seems to be largely reversible, but with significant hysteresis (the yellow-to-blue transition occurring at about 40 OC on cooling). The large hysteresis suggests sluggish kinetics for the color transition. It may be that the reorganization of the polymer chains in CHCl,/hexane is even more sluggish, so that the chains are trapped in a conformation controlled mainly by the solvent environment. We have also evaluated the integrated optical absorption (extending the integration to higher energies than those displayed in the figures). We find no significant variation with temperature (
