Temperature dependence of spectroscopic properties of isolated polydiacetylene chains strained by their monomer single crystal matrix
Introduction
The great sensitivity of optical properties to conformational changes is a pervading issue in the field of conjugated polymers. There is a need for model systems in which all chains in the sample have the same conformation, and in which the conformation can be modified in a controlled way. Polydiacetylenes (PDA) provide such a model system. Their general formula is (R–C–CC–C–R′)n where R and R′ are side groups, which may be different or identical, and may have very diverse chemical formulae. For several PDA, a polymer single crystal can be obtained by polymerization of a single crystal of the corresponding diacetylene (DA) monomer [1]. The initiation reaction is homogeneous and the transformation proceeds via a continuous series of monomer–polymer solid solutions with increasing polymer content.
In the present work, two DA known as 3BCMU and 4BCMU are studied. Their side-group formula is R = R′ = –(CH2)n–OCONH–CH2–COOC4H9 with n = 3 or 4 for 3BCMU and 4BCMU, respectively. These materials readily polymerize upon irradiation with UV, X or γ-ray photons, but show no significant thermal polymerization, so a small and constant chain concentration can be created and kept almost indefinitely. Polydiacetylenes show two different electronic structures, called “blue” and “red” [2]. In 4BCMU monomer all chains are blue, and in 3BCMU less than 10−3 of the chains are red. This work deals exclusively with blue chains. The first polymer chains formed at early stages of polymerization are individually dispersed in the monomer crystal matrix. They all have the same geometry and are embedded in the same periodic potential imposed by the matrix. In 3 and 4BCMU this is manifested by the small inhomogeneous broadening of absorption and emission lines, which are very narrow at low temperature [3], [4]. These chains may be very long: an average length of 6 μm has been measured for monomer crystals of 4 and 3BCMU [5]. The ensemble of chains in a monomer single crystal thus provides a very well ordered state, which may even be better ordered than the corresponding pure polymer crystal.
Since the polymer chains show strong absorption in the visible range, with a maximum absorption coefficient for light polarized parallel to the chain direction αmax ∼ 7 × 105 cm−1 to 106 cm−1 at 300 K [6], whereas the monomer is transparent up to ν = 3.55 × 104 cm−1 or 4.4 eV (Fig. 1), the chain spectroscopic properties can be studied down to very low polymer content, below 10−5 in weight. At such dilutions, the average distance between chains is larger than 100 nm, so there is essentially no electronic interaction between them. This system has been used as a good model for the study of electronic properties of conjugated polymers, since it is up to now the experimental system closest to theoretical models, and it has been shown that such isolated chains behave at low temperature as quasi perfect quantum wires [2], [3], [4], [7], [8], [9], [10], [11], [12], [13], [14], [15].
It may also be a good model system for studying the effect of a controlled deformation of the chain on the electronic properties of a conjugated polymer. In fact, it will be shown below that in this system, chain geometries not otherwise accessible are present.
In the case of chains dispersed in their monomer crystal, the repeat distance along the chain is dependent of the surrounding matrix unit cell parameters, so it can be varied in a controlled way by varying the temperature, 3BCMU and 4BCMU are studied here in that way.
The temperature dependence of absorption and resonance Raman spectra of polymer chains diluted in the monomer crystal are studied to probe the effect of known strains. The wavelength of maximum absorption gives the T dependence of the exciton energy, thus information on the effect of conformation on electronic structure. The resonance Raman spectrum yields the frequencies of phonon modes most strongly coupled to the π electrons, thus information on the ground state geometry.
These two monomers were studied because they have quite different behaviour as far as their crystalline structure is concerned [16]. In 3BCMU monomer crystal, the parameter corresponding to the chain direction is always close to the relaxed polymer value, the difference being less than 0.5% at all temperatures. In 4BCMU, the chains are already under ∼1% compression at 300 K, and the strain increases steadily up to ∼3% at 15 K.
In all experiments the content of the polymer chains embedded in the monomer matrix is kept very low (less than 10−4 in weight). the polymerization process is studied elsewhere [17].
The paper is organized as follows: experimental methods are presented in Section 2, spectroscopic properties (absorption, Raman) and a comparison between isolated chain and bulk PDA in Section 3, while Section 4 is devoted to the discussion, and a model of chain geometry changes under compression is proposed to explain the red shift of the exciton energy.
Section snippets
Absorption
A CARY 2300 double beam spectrometer equipped with polarizers is used, in the UV–visible range.
Raman
Excitation was produced by an Argon laser (Coherent Innova 90) pumping a dye laser (Coherent CR99) using Rhodamine 6G, Rhodamine 110 and DCM dyes. A continuous range of wavelengths from 560 to 680 nm is obtained. Below 560 nm the green lines of the Argon laser are used. The signal is dispersed in a double monochromator (Jobin Yvon Ramanor U1000) and detected by a cooled RCA C31034 photomultiplier. The
Results and discussion
The exciton energy E0 and the Raman frequencies vary with T quite differently in the two materials. These variations will be discussed in light of the structural changes described in [16].
Exciton energy variation with temperature
Any explanation of variations affecting the exciton energy E0 thermal shift and discontinuity at the phase transition of the monomer single crystal (Fig. 2), has to be consistent with the evolution of the double bond stretching νD with temperature (Fig. 5), which suggests a deformation of the chain in the ground state.
At the phase transition E0 increases while νD decreases, a torsion of the chain induces an increase of both E0 and νD so such a deformation can not explain the observations. We
Conclusion
We had two reasons to study the 3 and 4BCMU structures [16]. The structure governs the topochemical reaction; it also governs the geometry of isolated polymer chains in the monomer matrix, the periodic potential in which they are embedded, and hence their spectroscopic properties, which have been studied in details.
The 3BCMU structure shows that unstrained isolated chains are in register with the monomer matrix at all temperatures, and so are representative of chains in bulk PDA. Indeed, the νD
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