Water-Induced Tuning of the Emission of Polyaniline LEDs within the NIR to Vis Range

Tuning of the emission within the near-infrared to visible range is observed in p-toluenesulfonic acid-doped polyaniline light emitting diodes (PANI/PTSA), when water molecules are absorbed by the active material (wet PANI/PTSA). This is a hybrid material that combines a conjugated π-electron system and a proton system, both strongly interacting in close contact with each other. The proton system successfully competes with the electron system in excitation energy consumption (when electrically powered), thanks to the inductive resonance energy transfer from electrons to protons in wet PANI/PTSA at the energy levels of combination of vibrations and overtones in water, with subsequent light emission. Wet PANI/PTSA, in which electrons and protons can be excited parallelly owing to fast energy transfer, may emit light in different ranges (on a competitive basis). This results in intense light emission with a maximum at 750 nm (and the spectrum very similar to that of an excited protonic system in water), which is blue-shifted compared to the initial one at ∼850 nm that is generated by the PANI/PTSA dry sample, when electrically powered.