Triplet Exciton Dynamics in Fluorene−Amine Copolymer Films

Effect of aromaticity on triplet exciton dynamics was studied by transient absorption spectroscopy for two fluorene-based random copolymers with different aromatic amine, poly(9,9′-di- n -octylfluorene- ran - N, N ′-bis(4- n -butylphenyl)- N, N ′-diphenyl-1,4-benzenediamine) (F8-PDA) and poly(9,9′-di- n -octylfluorene- ran - N, N ′-bis(4- t -butylphenyl)- N, N ′-diphenyl-9,10-anthracenediamine) (F8-ADA). On a time scale of nanoseconds, triplet exciton was efficiently formed in F8-PDA through the intersystem crossing (ISC) from singlet exciton with a rate constant of 2.0 × 10 8 s –1 . On the other hand, the ISC was not efficient in F8-ADA, resulting in efficient fluorescence emission. On a time scale of micro- to milliseconds, F8-PDA exhibited bimolecular triplet exciton decay due to triplet–triplet annihilation (TTA), but the TTA was negligible in F8-ADA, indicating that triplet excitons can diffuse freely in F8-PDA while they are trapped at ADA units in F8-ADA. The difference in the ISC efficiency and triplet exciton diffusion is discussed in terms of the aromaticity of the amine units. On the basis of these analyses, we discuss a strategy for further improvement in the efficiency of fluorene–amine copolymer based light-emitting diodes.