Solution and Solid State Properties of 1,4‐Diketo‐3,6‐bis‐(4'‐pyridyl)‐pyrrolo‐[3,4‐c]‐pyrrole on Protonation and Deprotonation

Abstract Solution and solid state properties of 1,4‐diketo‐3,6‐bis‐(4'‐pyridyl)‐pyrrolo‐[3,4‐c]‐pyrrole (DPPP: red pigment) have been investigated with respect to protonation and deprotonation. DPPP undergoes both protonation and deprotonation, because it behaves not only as a base due to electron lone pairs of the nitrogen in the pyridyl ring but also as an acid due to the NH group in the heterocyclic chromophore. Protonation at the pyridyl nitrogen and deprotonation at the NH group induce large bathochromic shifts in solution. These are interpreted in terms of Pariser‐Parr‐Pople calculations, using the variable β, γ‐approximation. Protonation in the solid state due to acid vapor brings about significant changes in photoelectrical properties, such as a reduction in electrical resistivity from about 10 10 to 10 5 Ωcm and strong photoconduction enhancement. The protonated solid state has been characterized systematically through spectroscopy, crystallography and thermo‐analysis with particular attention to its potential applications, e.g., to proton sensors and optical information storage systems.