Polymorph of 1,4-diketo-3,6-bis-(4′-dipyridyl)-pyrrolo-[3,4-c]pyrrole and their hydrogen bond network: A material for H2 gas sensor

Diketodiphenylpyrrolopyrroles (DPP) are industrially important red pigments. The title compound (DPPP) is a dipyridyl derivative that possesses a high proton affinity because of the N atoms of the pyridyl ring. A drastic change in shade as well as electrical conductivities by several orders of magnitude is observed due to protonation at the N-atom. Therefore, DPPP has newly attracted attention as a material for H2 gas sensors. Then, we noticed that there are two crystal phases: one is quite sensitive to protons (phase I grown from the vapor phase) while the other is rather resistant to protonation (phase II recrystallized from solution). Therefore, structure analysis has been carried out in the present investigation in order to clarify the correlation between the structure and proton affinity. A striking difference is recognized in intermolecular hydrogen bonds between two phases: phase I is characterized only by NH⋯O hydrogen bonds where two N atoms of the pyridyl rings remain free. On the other hand, phase II includes two types of the H-bond based on NH⋯O and NH⋯N where the one N atom of the pyridyl ring is used by NH⋯N hydrogen bond and the other remains unused (i.e. free). Therefore, phase I is found to correspond to the phase which exhibits a high proton affinity while phase II is the one which is rather inactive for protons.