Butatriene structure

Polydiacetylenes are produced by the solid-state polymerization of single crystalline diacetylenes of the form RC=C—C=CR by 1,4-addition (see Fig. 32). The fully conjugated backbone of the polydiacetylenes is a model quasi one-dimensional electronic system being capable of representation by two extremes of bonding, sequence (a) an acetylene structure -fRC—C=C—CR)- and sequence (b) a butatriene structure -fRC=C=C=CR)-A,. There is great current debate as to the best method of representation but, in line with the belief that the nature of the side groups affect the bond lengths (and hence the... 

The experimentally most intensively investigated systems are PTS (R= CH2S03C6H4CH3) and TCDU [R=(CH2)4OCONHCeH5], the former with an acetylene-like backbone69 while the latter is best represented by the butatriene structure.70 The experiments performed on these and other PDA crystals involved photoconductivity measurements,71 Raman spectroscopic studies,72 investigation of the core- and valence-electron spectra by UPS and XPS,78 as well as by absorption and reflectivity measurements.74... 

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. 

At the ends of the polymer chains and at the ends of the short oligomer units (see for example the trimer molecule of Table 1) a bond defect structure is expected. For the acetylene structure of the polymer chain this is a carbene —C— with two free valence electrons and in the case of the butatriene structure this is a radical carbon atom —C= with one free valence electron. In both rases there is a reactive chain end, which allows reaction of the chain with the neighbouring monomer molecules. These reactive structures and a possible nonreactive structure are listed in Table 1 as examples of the trimer molecules. 

Owing to the low values of their fine structure parameters and to their characteristic hyperfine pattern the thermally activated triplet states I, II, III,. .. are identified as diradical DR oligomers with the following symmetric butatriene structure ... 

Butatriene structure (theor.) Acetylene structure (theor.)... 

Two different reaction mechanisms have been postulated for the topochemical polymerization of diacetylenes involving diradical or carbene chain ends The first mechanism leads to a butatriene structure (I), the latter to the acetylene structure (II) of polydiacetylene chains. 

The experimental data for various polydiacetylenes and for two model compounds for the butatriene structure are compiled in Table 4. 

FIGURE 8. Chain structures of polydiacetylene (i) acetylene structure, (ii) butatriene structure. 

Katagiri, H., Y. Shimoi, and S. Abe. 2004. A density functional study of backbone structures of polydiacetylene Destabilization of butatriene structure. Chem Phys 306 191. 

Little change in the energy of the exciton was found for n values greater than 10. The diradical oligomers (DR)n have the butatriene structure in idilch the exciton has a lower energy than in the acetylenic bonding sequence these were not found to be stable beyond... 

There is little information available concerning the cyclobutadienyl radical anion 4. Experimental evidence was obtained for the radiolytic formation of the acetylene radical anion in HZSM5 zeolite, and the structure and energy of 4, which is a possible product from this reaction, was calculated using DFT methods. A rectangular planar structure was found with CC bond lengths of 1.519 and 1.400 A, as compared to values of 1.509 and 1.381 A calculated for the radical cation 3. Linear butatriene structures CH2=C=C=CH2 were calculated to be lower in energy than 4 by 37—38 kcal/mol. 

Recently the theoretical models developed for polyacetylene have been applied to PDAs (00, 115, 116). A degenerate ground state does not occur for PDA so that this discussion is limited to polarons. The potential surface for the ground state structures deduced by this method is very similar to that calculated by the ab initio method (00, 106) indicating that the alternative butatriene structure is metastable. A further conclusion is that the band gap is primarily due to the bond alternation produced when the TT-electrons are added to the bare PDA backbone. These calculations are currently being extended to make realistic exciton calculations for PDAs (117). The occurrence of acousticmode polarons and exciton-polarons in PDAs has also been discussed recently (95, 145). 

---