Temperature-dependent photoluminescence of organic semiconductors with varying backbone conformation

We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule all with distinctly different backbone conformations. The organic materials investigated here are planar methylated ladder-type poly para-phenylene, semi-planar polyfluorene, and nonplanar para-hexaphenyl. In the long-chain-polymers the photoluminescence transition energies blueshift with increasing temperatures. In contrast, in the conjugated molecules transition energies redshift with increasing temperatures. Empirical models that explain the temperature dependence of the band gap energies in inorganic semiconductors are extended to explain the temperature dependence of the transition energies in conjugated molecules.