Doping-Induced Immobile Charge Carriers in Polyazomethine: A Spectroscopic Study

Herein, we report the effect of protonation on the electronic properties and the conductivity of a soluble derivative of polyazomethine, poly(1,4-(2,5-bisoctyloxy phenylenemethylidynenitrilo)-1,4-phenylene-nitrilomethylidyne), BOO-PPI. Following protonation, a marked change in electronic structure occurs, as evidenced by a bathochromic shift in visible absorption and the appearance of a broad absorption in the infrared coupled with new infrared active vibrational modes, signaling the appearance of a new electronic state. In other conjugated polymers, such features usually evidence the formation of mobile polarons on the polymer chain, leading to an increase in electrical conductivity. Surprisingly, BOO-PPI protonation does not lead to such an increase. Protonation also does not result in an electron spin resonance signal. We propose a model for protonation inducing a charged state on the polymer chain with strongly limited delocalization accounting for the lack of the electron spin resonance signal and the missing conductivity. To our knowledge, this is the first time that spectral features usually corresponding to mobile charges in organic materials do not correspond to an actual change in conductivity.