Charge-transfer character of excitons in poly [ 2 , 7 - ( 9 , 9 -di- n -octylfluorene ) ( 1 − x ) -co- 4 , 7 - ( 2 , 1 , 3 -benzothiadiazole ) ( x ) ]

Quantum-chemical calculations performed on poly [ 2 , 7 - ( 9 , 9 -di- n -octylfluorene ) ( 1 − x ) -co- 4 , 7 - ( 2 , 1 , 3 -benzothiadiazole ) ( x ) ] copolymers ( 0 ≤ x ≤ 0.5 ) show that the lowest unoccupied molecular orbital is always highly localized on the benzothiadiazole (BT) units while the highest occupied molecular orbital is delocalized over the whole chain. Chains with a low BT content are characterized by a reduced oscillator strength of the lowest optical transition and by an increased charge-transfer character of the exciton. These results are supported experimentally by a blueshift of the lowest energy absorption band upon reduction in the BT ratio, lower photoluminescence efficiency, longer excited state lifetimes, and greater solvent dependence of the emission properties.