Control mechanisms for transport and nonlinear optical response in organic materials: a tale of twists and barriers

Simple electronic structure models are used to address two significant challenges in organic materials chemistry, the design of chromophores for strong electro-optic response (and low-energy optical absorption), and the prediction of relative mobilities and charge injection barriers for conductive oligomers. For electro-optic response, we examine two chromophore classes where twisting around an inter-ring bond can tune the electronic structure from aromatic (zwitterionic) to quinoid (neutral). The calculated nonlinear response develops a very strong maximum (βμ ∼ 1500 ×10−30 esu) at twist angles near 80°. For the transport behavior, structure/function correlations are presented for three series of oligomers, based on calculations of bandwidths (as functions of geometry) and of reorganization energies. Transport type appears to be fixed less by these mobility factors than by the injection barriers. The simplest estimates for these Schottky-type barriers, using frontier orbital energies from density functional calculations, predict carrier n-type or p-type behavior remarkably well.