Absorption studies of charged excitations in α-sexithiophene

We present photoexcitation studies of vacuum deposited neutral films of α,ω-substituted sexithiophene (α-6T) using photoinduced absorption (PA) and PAdetected magnetic resonance spectroscopy (PADMR). We find evidence for photoinduced polarons having spin 1/2, with two absorption bands at 0.80 and 1.54 eV, respectively, and with negative PADMR signal at g ≈2. Similar absorption bands are observed in lightly p-doped sexithiophene and are interpreted as due to 6T*+ radical cations. In addition, a PA band is found at 1.1 eV, which is shorter lived and decreases faster with increasing temperature than the polaron bands. As similar dication bands (6T2+) have been found in heavily-doped α-6T, we identify this band as due to spinless bipolarons. Evidence of charge conjugation symmetry breaking in α-6T is presented. Finally, we have identified triplet excitons with triplet-triplet transition energy at about 1.45 eV.