Raman shifts due to variations in backbone planarity in a group of polydiacetylenes

✍ Scribed by Alasdair J. Campbell; Craig K. L. Davies; David N. Batchelder

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 390 KB
• Volume: 199
• Category: Article
• ISSN: 1022-1352
• DOI: 10.1002/(sici)1521-3935(19980101)199:1<109::aid-macp109>3.0.co;2-n

✦ Synopsis

Polydiacetylene xBCMU and 9PA crystals, solutions and solution-cast films vary in colour due to differences in backbone order and planarity. Associated with the colour changes are increases in the Raman frequencies of the triple and double backbone bond vibrational modes from those values found in the highly ordered single crystals. These increases are strongly correlated and the ratio of the change in the frequency of the triple bond mode to that of the double bond mode is found to be 1.6 (iO.l) over a wide variation in order. This ratio is compared with the values of 0.3 found under uniform strain and 1.1 predicted by a model of acetylenic to butatrienic delocalisation. The relationship between the Raman frequencies of the yellow solutions and melts and those of the crystal phases suggests that in the former the backbone has a continuous and smoothly curving 'worm-like' conformation. Models involving the break-up of the conjugated system by large-angle bond rotations appear to be incorrect for polydiacetylenes. These results have important consequences for theoretical models that predict the dependence of electron delocalisation on backbone order and planarity in polydiacetylenes.