Dialkyl Side Chain Substituents

Much of current conjugated polymer research is directed at two major technology thrust areas, better control of charge transport and enhanced photophysics. The requirements for latter, in terms of molecule structure, are very much application dependent. For the simplest electroluminescent (EL) devices, LED's, increasing EL yield, tuning of the interband transition and minimizing inhomogeneities are typical design goals. A common motivation for attaching branched or asymmetric substituents is simply to introduce local disorder at molecular length scales. This tends to suppress crystallization and also to minimize cofacial contact (i.e., aggregation) with neighboring chains. In this way $p_z$ wavefunction overlap is diminished and interchain electronic excitation processes are reduced. In principle this should lead to higher EL yields in the solid state. Reducing or eliminating crystallization is thought to generate more uniform distributions of conjugation lengths and provide emission properties which are less sensitive to physiochemical processing and aging effects. Increasing molecular level disorder may improve device behavior in very specific cases but, in general, this will conflict with studies addressing more fundamental issues.

Of the many possible candidates we discuss just two polymer families, poly(di-alkylsilanes) and poly(di-alkylfluorenes), because they are known for high photoluminescence and EL yields and, additionally, these polymers can exhibit exceptionally well ordered crystalline structures and an extensive polymorphism. The existence of crystalline phases represents an important reference frame for investigating the fundamental properties and characterizing the impact that increased disorder has on device behavior and performance. Moreover both may be viewed as prototypical examples of electronic polymers incorporating more complex side chain substituents; in this case dialkyl side chain moieties.

Winokur 2004-01-28