Since the first report of polymeric light-emitting diodes (PLED) based on poly(p-phenylenevinylene) (PPV) by the Cambridge group in 1990

great progress has been made in PLEDs due to their promising applications in the field of patterned light and flat panel display. Single-color and full-color have been realized. At present

the main work is to improve the performance of PLED with high quantum efficiency

high luminescence and long life. But it’s very difficult to get these because the properties of these functionalized light-emitting polymers can be easily modified by external stimuli such as temperature

pressure and solvent

etc

. Though major effort has been focused on determining the relationship between polymer chemical structure and conformation and the corresponding physical and chemical properties

there is not still good theory to direct us to utilize the conformational transition and the effects of the aggregation. In this work

we focus on the effect of the aggregation and conformation of functionalized light-emitting polymers on their luminescent properties. The materials we used here are poly[(2

5-didecyloxy-1

4-phenylene) vinylene] (DD-PPV) and poly[2-methoxy-5-(2'-ethylhezyloxy)-1

4-phenylene-vinylene]

(MEH-PPV). The UV-Vis absorption peaks of MEH-PPV centered at 494nm in chloroform solution and 504nm in the drop cast film. And its maximum emission has a red-shift from 555nm in solution to 603nm in solid state. The red-shift of absorption from solution to solid state is commonly observed and usually is attributed to the aggregation in the solid state. Moreover we investigated the UV-Vis absorption features of MEH-PPV in the mixtures of chloroform (good solvent) and methanol (poor solvent). Upon increasing the amount of methanol

an evident color change was observed

e.g. the red shift of the maximum of absorption from 495nm to 548.5nm

which indicates that a more conjugated and ordered structure appears upon aggregation. MEH-PPV in solution might have two distinct conformational structure

a co-planar (or nearly planar) conformation and the twisted structure. The presence of an isosbestic point confirms our hypothesis. However

blue shift of the maximum absorption of DD-PPV was observed in the same mixtures. Perhaps it’s because there is only one kind of conformation. So the poor solvent methanol decreases the conjugated length of this two polymers.