The solvatochromic effect on some oligothiophene radical cations: a pulse radiolysis and semiempirical investigation

The characteristic UV-vis absorption peaks of ter-, quater-, and quinquethiophene radical cations have been investigated by pulse radiolysis in CCl4, CHCl3, and CH2Cl2. A band shift ranging from 5 to 25 nm towards longer wavelengths is evidenced, as dielectric constant e increases. Electronic transitions are reasonably reproduced by semiempirically optimized structures. The related dipole moment variations between ground and excited states led to justify the spectral shifts envisaging that radical cations in solution consist of a mixture of conformers with a prevailing ratio of syn- with respect to anti-conformers. Referring to the all-syn-conformer radical cations, dipole moments go from 2.8 to 5.2 D (ground states), and from 7 to 9.3 D (excited states), growing with the number of rings. The Onsager radii fit expected values for aromatic molecules, corresponding to 6.2 Å (3T+·), 6.0 Å (4T+·), and 7.7 Å (5T+·).