Twisting inter-ring deviation from planarity in oligo- and polythiophenes

5 Twisting (inter-ring deviation from planarity) in oligo- and polythiophenes 

Elmaci and Yurtsever used DPT to study the relationship between thermochromism and internal rotation in poly thiophenes [59b]. For head-to-tail methyl-substituted thiophenes, rotational potential maps of the dimer and trimer were calculated. Using a time-dependent DFT methodology, the UV-Vis absorption spectra of these oligomers were calculated as functions of the torsional angles. Theoretical temperature-dependent UV-Vis spectra were generated from the distribution of torsional angles. The results show that the internal rotation alone is not sufficient to explain the mechanism of thermochromism. 

It is also found that the thermochromic transition temperature for poly(3-alkylthiophene) thin films decreases as the length of the alkyl side-groups increases [71]. If the main chain is distorted by either twisting or folding, the alkyl chains take mn-trans conformations. This is explained as a transfer of thermal energy from the side-chains, where it is stored, to the polymer skeleton, which then breaks the stiffness of the backbone and decreases the r-conjugation length. 

Di Cdsare et al. studied the lowest energy structures of several bithiophenes substituted with methoxy and methyl groups in various positions using HF/3-21G calculations, and also performed a detailed conformational analysis [74]. The effect of the position of substituents on structural parameters and on torsional potentials was examined. The results of these calculations are in good agreement with available experimental data. 

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