Substitution longer oligothiophenes

Longer oligothiophenes can only be studied in solution when they are alkyl-substituted. Dimethyl-oligothiophenes (DManT) with = 4,6,8,10 were studied by Zotti et at. [205]. For DMa4T they found... 

The crystalline and molecular structure of several substituted oligothiophenes have been reported. Substitution may have a two-fold effect i) to make the longer oligomers more soluble and therefore more tractable for applications, and ii) to provide insight about the steric effects of the substituents for the molecular (and also the polymer) configurations. 

Here a complete overview is given on the structure of thin films of quinque- and sexithiophene, whereas films of smaller, longer, and substituted oligothiophenes are only stressed briefly. 

It is commonly accepted that in solution a-6T is oxidized in two steps, first to the radical cation, then to the dication. The cationic state is believed to exist as a 7r-dimer in which two radical cations couple via their 7r-systems and which has a nearly spin-less state [292-297]. This u-dimer forms as easier as longer the chain length and as lower the temperature is due to its exothermic building process. ESR measurements show that the mono-oxidized oligothiophene is a (nearly) free radical with an intense and narrow Lorentzian signal centered at g = 2.0023-2.0025 for unsubstituted [298] as well as alkyl-substituted a-nT [293], The dication formation is only possible for a-6T and longer oligomers, q-4T and a-5T either do not react or dimerize to octi- and decithiophene, respectively [299]. (For a review on results obtained in solution, compare [250].)... 

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