Appendix I:

The centrosymmetric title compound, C16H10N4O2, is an organic red pigment utilized for H2 gas sensors. The centrosymmetric diketopyrrolopyrrole moieties are connected by bifurcated N-HO hydrogen bonds to form a ribbon structure along [110] and [110] alternately. The molecules are stacked in a “hunter's fence” (viz. when viewed from the side, molecules, slipped by 45° within molecular stacks, cross each other in a fence-like structure) fashion along the a axis. Figure 1 A view of the molecular structure of (I), showing 30% displacement ellipsoids for non-H atoms. The unlabeled atoms are related by the symmetry code (1-x, 1y, 1-z). (I) Comment Diketodiphenylpyrrolopyrroles are industrially important red pigments (Herbst & Hunger, 1993). The title compound, (I) (p-DPPP), is a dipyridyl derivative that possesses a high proton affinity because of the N atom of the pyridyl ring. A drastic change in color as well as electrical conductivities have been observed due to protonation at the N atom (Mizuguchi, 1993). Because of this, p-DPPP has newly attracted attention as a material for