Oligothiophene dications

Finally, it would be pertinent here to demonstrate that the present methodology of cation stabilization can be successfully applied to the preparation of a series of radical-cation salts and dication salts of oligothiophenes (2T, 3T, 4T, and 6T), fully annelated with BCO units, 24, 25, 26, and 27 (Figure 18) (31,32). 

Radical cations and dications of oligothiophenes have been investigated by many research groups to understand the nature of polarons and bipolarons in polythiophenes which are responsible for the electrical and optical properties of these re-conjugated polymers. 

The neutral ( T), radical cation ( T ), and dication ( T ) species of a-oligothiophenes with increasing number of rings (n = 2, 4, 6, 8, 10, and 12) (Figure 5) have been considered by means of DFT. These results are important in that they significantly differ from those obtained at the lower HF-SCF theory level <1994JPC7492>. 

Using TD-DFT at the B3LYP/6-31G(d,p) level to calculate the electronic transition yields a band at 1.68 eV (/ = 1.77) for H4T4CN4, which corresponds well with the experimental band at 790 nm (1.57 eV) with a shoulder at 871 nm. No other electronic transition with a significant intensity was calculated below 3.25 eV. This band arises from HOMO-LUMO excitations. The energies are in fact similar to those observed for aromatic oligothiophenes in the dication or bipolaron state [37b, 99], for which quinonoid... 

Other a,co-disubstituted oligothiophenes such as 31 [24], 32 [103], 33 [104], 34 [105], 35 [106] and 36 [98] were characterized by cyclic voltammetry (Scheme 9.23). In 35, bulky bicyclo[2.2.2]octene substituents were introduced to prevent dimerization. Indeed, all oligomers derived from 35 (n = 0, 1, 2, 4) show stable radical cations and dications. In the case of the hexamer derived from 35 with n = 4, the two first one-electron reversible oxidation waves are nearly coalesced ( ox 0. 19, = 0.28 V vs FC/FC+)... 

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