Diacetylene crystals

Sixl, H. Spectroscopy of the Intermediate States of the Solid State Polymerization Reaction in Diacetylene Crystals. Vol. 63, pp. 49 — 90. 

Figure 4. Reflectivity of the PTS-diacetylene crystal for two different temperatures.

As an example of resonance CARS studies in solids, for which a linear resonance Raman study has been impossible to perform because of simultaneous strong luminescence, we consider here investigations on colour zones in substituted diacetylene crystals... 

Hoofman RJOM, Siebbeles LDA, de Haas MP, Hummel A, Bloor D. (1998) Anisotropy of charge-carrier mobility in poly diacetylene crystals. J Chem Phys 109 1885-1893. 

The latter effect can be explained by taking into account that a partially polymerized diacetylene crystal is a molecular crystal containing dopant molecules with lower lying optical transition acting as traps for monomer excitations . Take k as the rate constant for non-radiative energy transfer from a donor to a trap, Tq as the intrinsic donor lifetime, and c, as the relative trap concentration, then the lifetime of a donor in presence of traps would be T = -I- k cj Since the probability that an excited... 

Although being insulators in general at least one crystalline polydiacetylene DCH was found to be electrochemically dopable up to a conductivity of 10 (Qm) by applying an NaJ/J2 electrolytic contact One could think of combining this technique with electron-beam polymerization in order to produce a well-defined conducting pattern at the surface of a diacetylene crystal support or LB system. 

The polymerization of the diacetylene crystals is usually described by the following reaction equation... 

Fully conjugated and fully chain-aligned polymer single crystals with planar polymer backbone are obtained, which may have the alternative acetylene (ynene) or butatriene structures of Eq. (1). From our experiment we know that the acetylene structure is dominant in the polymer molecules. Up to now the best investigated diacetylene crystals are the TS-6 monomer crystals and the corresponding polymer crystals (poly TS-6). The substituents R and the notation of further diacetylene crystals discussed below are listed in Table 1. 

A direct measure of the optical absorption coefficient a is the optical density OD defined by OD = log Iq/I = 0,434od (Iq and I are the incident and the transmitted light intensities and d is the thickness of the crystal). Weak polymer absorption in the range from 600 to 400 nm is present in the original monomer crystals due to weak thermal polymerization reactions. The absorption of the linear polymer molecules (which are homogeneously distributed within the partially polymerized monomer diacetylene crystals) increase during UV-irradiation at room temperature due to photopolymerization reactions. In contrast to the monomer absorption, the polymer... 

Fig. 3. Optical absorption spectra of a diacetylene crystal (TS-6) at about room temperature. The optical density is proportional to the absorption coefficient. The absorption in the visible spectral region is due to the polymer filaments. UV photopolymerization has been performed in steps using the same photon numbers, respectively...

In all experiments described in this work only extremely low concentrations of intermediates are considered. This is due to our interest which is primarily focussed on the most important initial steps of the polymerization reaction, which are characteristic of the overall polymerization reaction mechanism. Consequently only low final polymer conversion is exp>ected and, therefore, complications arising from the interaction between the intermediate oligomer states can be neglected. It will be shown that the low temperature conventional optical absorption and ESR spectroscopy are powerful spectroscopic methods which yield a wealth of information concerning structural and dynamical aspects of the intermediate states in the photopolymerization reaction of diacetylene crystals. Therefore, this contribution will center on the photochemical and photophysical primary and secondary processes of this... 

Fig. 4a and b. Optical absorption spectra of a diacetylene crystal (TS-6) at low temperatures, a Spectrum of the low temperature photoproducts obtained after UV-irradiation b Spectrum of the polymer filaments obtained after thermal polymerization at 300 K at low degree of conversion (about 100 ppm)... 

Fig. 8. Low temperature photoproduct absorption in diacetylene crystals with different substituents...

The observed time evolution and the decay of the photoproduct absorptions by photochemical and/or thermal reactions in the TS diacetylene crystals are described quantitatively by Eqs. (15) to (18). This has been demonstrated by Hersel Neumann , and Niederwald Some examples of calculated curves are shown in Figs. 5, 10, and 12. 

Fig, 24. Energy level diagram of the photophysical and photochemical primary processes of the low temperature photopolymerization reaction in diacetylene crystals. IC Internal Conversion ISC Intersystem Crossing... 

In recent experiments the photopolymerization process has been initiated with visible light. Sensibilization of the photopolymerization reaction is possible in diacetylene crystals by introduction of energetically low lying absorptions of the substituents via formation of mixed crystals or by doping with dye molecules Although the detailled mechanism of the sensibilization is not clear, the experiments clearly demonstrate the importance of lowlying electronic states in the polymerization reaction. 

RC = C=C=CR-]-, which have the same total number of c- and jt-bonds. An energy difference of about 0.4 eV per unit easily could explain the observed structural change at about n = 6. This butatriene-to-acetylene transformation has been observed also in the TS-12 and TCDU diacetylene crystals at about the hexamer unit. 

Owing to the bimolecularity of the initiation reaction the quantum yield of the dimer molecules (M2/Nji ) is proportional to the absorbed light quanta N bs and to the ratio kj/ko, characterizing the competition of the chemical dimer initiation process (kj) with the deactivation processes (ko) of the monomer excitation. A comparison of the dimer A absorption intensities of different diacetylene crystals shows that the ratio kj/ko is about a factor of 10 to 10 larger in the TS-6 crystals than in... 

In this article it has been shown, that the low temperature photopolymerization reaction of diacetylene crystals is a highly complex reaction with a manifold of different reaction intermediates. Moreover, the diacetylene crystals represent a class of material which play a unique role within the usual polymerization reactions conventionally performed in the fluid phase. The spectroscopic interest of this contribution has been focussed mainly on the electronic properties of the different intermediates, such as butatriene or acetylene chain structure, diradical or carbene electron spin distributions and spin multiplicities. The elementary chemical reactions within all the individual steps of the polymerization reaction have been successfully investigated by the methods of solid state spectroscopy. Moreover we have been able to analyze the physical and chemical primary and secondary processes of the photochemical and thermal polymerization reaction in diacetylene crystals. This success has been largely due to the stability of the intermediates at low temperatures and to the high informational yield of optical and ESR spectroscopy in crystalline systems. 

A polymerizing diacetylene crystal can be considered as a composite material with large differences in the mechanical properties of both components. In such a material the mechanical properties will not only depend on the relative amount of the components but also very strongly on the geometrical arrangements of the structural elements Two limiting cases can be considered The first model consists... 

Bloor, D. The polymerization of disubstituted diacetylene crystals. In Developments in crystalline polymers. Bassett, D. C. (ed.). Barking Applied SciencePubUshers 1982, pp. 151-193... 

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