Intrachain confinement of excitons and radiative recombination in polyaniline grains

Suppression of the benzenoid-to-quinoid excitonic transition (BQET) and enhancement of the quantum yield of photoluminescence (PL) at ∼390 nm has been observed in grained films of the emeraldine base (EB) of polyaniline. The major PL band from the EB films with the grained morphology was relatively narrow, with FWHM of about 1020 cm−1, followed by vibronic overtones the intensity of which was dependent on the mode of the PL excitation. The change of the film morphology, where instead of grains large aggregates were formed, leaded to the recovery of the absorption band corresponding to the BQET and quenching of the emission, respectively. It is suggested that the origin of PL in EB films is due to the distorted polymer chains in EB grains where the ring torsion angle of benzenoid group in respect to the adjacent quinoid group increases, thus resulting in the suppressed BQET and enhanced emission.