Oligothiophene radical cations

Recently Egelhaaf et al produced oligothiophene radical cations on silica gel by two-photon ionization with laser intensities I > 5x 106 W cm and laser flash energies above 15 mJ [38]. The electron is excited into a continuum state and no anion is formed. The absorption spectra are very similar to those observed for radical cations in solution. The ESR signal exhibits a signal with, g = 2.0028 at room temperature. At T= 77 K an anisotropic ESR spectrum is found. 

Quantitative investigations of the kinetics of these polymerization a-coupling steps have been performed by fast-scan voltammetry on ultramicroelectrodes in order to estimate the lifetime of the transient species. The rate constants for the dimerization of thiophene are greater than 10 M s, while the lifetime of oligothiophene radical cations increases with chain length the rate constant for unsubstituted thiophene tetramer is close to 10 and that of the pentamer is below 10 M s [18]. These studies have... 

Conversely, due to the defined structure of a-oligothiophenes it was quickly realized that they constitute interesting models of the electronic properties of polythiophene [32]. The first spectral characterization of oligothiophene radical cations and dications in 1989 [32c, 33] and the use of a-sexithiophene 6 as active component in an a//-organic electronic device one year later [34] have triggered a renaissance of interest and intensive research work in the synthesis and characterization of these materials that had been neglected for so many years [35]. 

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>. 

Oligothiophenes have been selected as the target molecule, since their photoexcitation processes are well established and they are known as electron donors with large extinction coefficients of the Tj state and radical cation in the D0 state [105-108], The S0 and Tj states of the trimer, tetramer, and... 

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