Abstract
Intrinsic optical transitions of lower Davydov excitons in thiophene/phenylene co-oligomer (TPCO) crystals were investigated. Lower Davydov excitons are either optically allowed or forbidden depending on the number of thiophene rings constituting the TPCOs. The TPCO molecules are orientated nearly parallel to each other like aggregates and are aligned in herringbone fashion within a crystalline layer. The transition selection rules are dominated by the components of the molecular transition dipole moments lying in the herringbone planes of the respective crystals. The selection rules at the exciton band bottoms of the TPCOs differ from those of oligothiophenes in spite of these two compounds having similar molecular structures. The selection rules vary drastically with the molecular alignment because of the aggregation. TPCO and oligothiophene crystals are treated as aggregates as a first approximation. This treatment is very useful for understanding the optical transitions of conjugated materials.
- Received 4 October 2007
DOI:https://doi.org/10.1103/PhysRevB.77.045205
©2008 American Physical Society