Polydiacetylenes

Crystalline polydiacetylenes, exhibiting a fully conjugated backbone, can be obtained [113] by thermal or photochemical polymerization of the monomer R C=C C=C R, where R is the organic substituent. The polymeric structures of the type 

As it was pointed out earlier, the optical and photoelectrical properties of the polydiacetylenes may be explained in the framework of ID crystal model in which the interaction between the electrons in the chain is much stronger than between the chains. The interaction energy differs by 100 fold. Considering the anisotropy of the optical and photoelectrical properties one can expect this fact. This was actually observed for monocrystals and films of the polydiacetylene. Absorption spectra of the monocrystal films are presented in Fig. 21 for varying light polarization [141]. 

Many authors have investigated the photoconductivity of the polydiacetylenes [142-171], The main problem discussed concerns the nature of the initial act of the photoeffect. At first, most authors considered the exciton formation to occur at the beginning with consequent dissociation on the free carriers. Then it was shown the broad band existence for directions along the chains. The unification of the excitonic and band model of the free charge carrier generation was developed [146-150], 

The photoconductivity and absorption spectra of the multilayer polydiacetylene are shown in Fig. 22 [150]. The continuous and dotted line relate to the blue and red polymer forms respectively. Interpretation is given in terms of a valence to conduction band transition which is buried under the vibronic sidebands of the dominant exciton transition. The associated absorption coefficient follows a law which indicates either an indirect transition or a direct transition between non-parabolic bands. The gap energies are 2.5 eV and 2.6 eV for the two different forms. The transition is three dimensional indicating finite valence and conduction band dispersion in the direction perpendicular to the polymer chain. 

An increase of the photocurrent at energies less than 2 eV was observed [151,152] unlike the previous result. This was attributed to the localized impurity ionization up to 0.8 eV below the conduction band. The crystals are considered as model systems for the one and three-dimensional versions of Onsager s theory of germinate recombination. 

Substituted polydiacetylenes form a unique class of conjugated polymers. A decade ago substituted polydiacetylenes have been solubilized and shown various color transition either by changing the solvent quality or the temperature. This has been the origin of a growing intoest on the solution of conjugated polymers (17). 

A very specific property of PDA is the polymerization route it is performed in solid 

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Batchelder D N, Evans S D, Freeman T L, Haussling L, Ringsdorf H and Wolf H 1994 Self-assembled monolayers containing polydiacetylenes J. Am. Chem. See. 116 1050-3... 

Rotations around torsional barriers induce changes in chain conformation. For conjugated systems like polydiacetylenes, flow-induced changes in chain conformation can have a profound influence on the photon absorption and electronic conductivity properties of the material [73]. Flow-induced changes in molecular conformation form the basis for several technically important processes, the best known examples are the production of oriented fibers by gel spinning [74], the compatibility enhancement [75] and the shear-induced modification of polymer morphology [76]. 

Finally, synthetic metals made of polymeric organic molecules may also show the property of ferromagnetism. Organic materials of this kind were first demonstrated in 1987 by Ovchinnikov and his co-workers at the Institute of Chemical Physics in Moscow. The polymer they used was based on a polydiacetylene backbone, which contains alternating double-single and triple-single bonds between the carbon atoms of the molecule (10.2). 

Bloor D, Chance RR (eds) (1985) Polydiacetylenes synthesis, structure, and electronic properties. Martin us Jijhoff, Boston... 

Polydiacetylenes which constitute an important class of polenynic polymers can be synthetized photochemically in the solid state from substituted diacetylenes. Experimental... 

A further partihon system based on the use of liposomes, and commercialized under the name Transil [110, 111], has shown its utiUty as a UpophiUcity measure in PBPK modeling [112]. Fluorescent-labeled liposomes, called fluorosomes, are another means of measuring the rate of penetration of small molecules into membrane bilayers [113, 120]. Similarly, a colorimetric assay amenable to HTS for evaluating membrane interactions and penetrahon has been presented [116]. The platform comprises vesicles of phospholipids and the chromahc Upid-mimehc polydiacetylene. The polymer undergoes visible concentrahon-dependent red-blue transformahons induced through interactions of the vesicles with the studied molecules. 

It is also shown that the electron-phonon interaction is operative in the polymerization process of the one-dimensional conjugated polymeric crystals a simple dynamical model for the polymerisation in polydiacetylenes is presented that accounts for the existing observations. 

The polydiacetylene crystals (1-4) most strikingly corroborate these conjectures. Along this line of thought is also shown that this electron-phonon interaction is intimately interwoven with the polymerisation process in these materials and plays a profound role there. We make the conjecture that this occurs through the motion of an unpaired electron in a non-bonding p-orbital dressed with a bending mode and guided by a classical intermolecular mode. Such a polaron type diffusion combined with the theory of non radiative transitions explains the essentials of the spectral characteristics of the materials as well as their polymerisation dynamics. ... 

Polydiacetylenes (2) come closest to the model one-dimensional organic semiconductor and can be readily obtained in form of large, nearly defect-free single crystals so that a large number of experiments and measurements have been carried out on these materials. Their structure is shown in Figure 3 where also some typical side-groups R are indicated. On table I we summarize some measured va-... 

Polydiacetylene crystals. The enhancement of x because of one-dimensional electron delocalization is strikingly corroborated in the polydiacetylene crystals. Their structure is that of a super alternated chain with four atoms per unit cell and the Huckel approximation yields four bands for the ir-electrons, two valence and two conduction bands. When depicted in the extended Jones zone, each pair can be viewed as arising by a discontinuity at the middle of the Brillouin zone of the polyene chain. The dominant contribution to X(2n 1) comes from the critical point at the edge of the extended Jones zone (initially at the center of the reduced B.Z.). The complete expressions are derived in (4,22) and calculated for different polydiacetylenes. We reproduce the values of x 2 for TCDU and PTS in table IV. The calculated values are in good agreement... 

On the other hand in the exciton description occasionnally adopted by some authors to interpret the main absorption peak in the polydiacetylenes one finds x " negative and its values two orders of magnitude lower than expression 6 since electron correlation (28) is essential in the exciton model, the calculation of even the simplest optical properties becomes prohibitively complicated and the physical insight is obscured. 

Because of this degeneracy and coexistence of these resonant structures one expects intrinsic conjugation defects to exist and be formed along the chain, the so-called Pople-Wamsley defects (34) which in the case of polyacenes and polydiacetylenes takes the form... 

The above conclusions introduce intrinsic limitations to the use of the ID conjugated systems in nonlinear optical devices. Although these may benefit (38) from the high nonlinearities,their response speed will be limited by the motion of such defects. These may also be formed by other means than light and this will clearly have implications on photoelastic, pyroelectric and piezoelectric effects as well. We point out that materials like polydiacetylenes may show appreciable quadrupolar pyroelectric effect (39). 

Here we outline a dynamical description (42) of the polymerisation of the polydiacetylenes. The approach relies much on the one used (43,44) in the theory of non radiative transitions in crystals and the soliton description of the defects in the lD-or-ganic semiconductors. 

The nonlinear optical properties of novel, soluble polydiacetylenes are reviewed and discussed. 

Recently, polydiacetylenes have been discovered which are soluble in common organic solvents (12). These polymers have urethane substituent groups of the form... 

This paper will review the linear and nonlinear optical properties of polydiacetylenes with an emphasis on our work with the nBCMU polymers. The following section will discuss material... 

Figure I. Unear absorption spectra for polydiacetylene solutions. The three solutions are referred to in the text as blue, red, and yellow (from left to right in the figure). (Reproduced with permission from Ref. 24. Copyright 1979, American Institute of...

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