Copolymerization is one of effective approaches for preparation of high performance emitting conjugated polymers.The emission color of polyfluorene can be tuned over the entire visible region by different(incorporating) narrow-band-gap(NBG) comonomers into the polyfluorene backbone.So photophysical properties of fluorene-based copolymers strongly depend on NBG comonomers.Based on comparative studies for optoelectronic performance of copolymers(PFDNT) of 9

9-dioctyfluorene(DOF) and 4

7-di(thien-2-yl)-2

1

3-naphthothiadiazole and copolymers(PFHDNT) of 9

9-dioctyfluorene(DOF) and 4

7-di(4-hexylthien-2-yl)-2

1

3-naphthothiadiazole

it has been found that the photoluminescence efficiencies of copolymer PFHDNT are significantly enhanced due to incorporation of alkyl chain into NBG heterocycle comonomers reducing interchain interaction of copolymer PFHDNT.In comparison with copolymer PFDNT without alky substitution on both thiophene rings

copolymer PFHDNT shows blueshift photoluminescence(PL) and electroluminescence(EL) emission at the same copolymerization ratio as a result of the introduction of the long alkyl chain into the thiophene ring enhancing steric hindrance and reducing the energy of π-π

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transition. Inaddition

the alkyl side chainin the 4 position of both thiophene rings can increase the NBG comonomer solubility in copolymerization reaction

thus themolecular weight of the resulted copolymers could be improved. On the other hand

the electrolum inescent efficiencies of those copolymers are not reduced because of incorporating alkyl chain into the NBG comonomers.