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Spectral signatures of polarons in conjugated co-polymers

: Wiebeler, C.; Tautz, R.; Feldmann, J.; Hauff, E. von; Como, E. da; Schumacher, S.


Journal of physical chemistry. B 117 (2013), Nr.16, S.4454-4460
ISSN: 1089-5647
ISSN: 1520-6106
ISSN: 1520-5207
Fraunhofer ISE ()

We study electronic and optical properties of the low-bandgap co-polymer PCPDT-BT (poly-cyclopentadithiophene-co-benzothiadiazole) and compare it with the corresponding homo-polymer PCPDT (poly-cyclopentadithiophene). We investigate the linear absorptivity in these systems for neutral molecules and for their singly charged ions based on quantum-chemical calculations and experiments. One of our main findings is that the ions of the homo-polymer show a polaron absorption that is symmetric between anion and cation, whereas for polaron excitations in the co-polymer this symmetry is strongly lifted. We demonstrate that this asymmetry can be attributed to the absence of symmetry between the high-lying occupied and low-lying unoccupied molecular orbitals in the co-polymer with type-II orbital alignment between the moieties constituting the chain. This notion is of importance for the qualitative and quantitative interpretation of spectroscopic polaron data and is not specific t o the system studied here but similarly applies to other co-polymers.