2006 Volume 45 Issue 4 Pages 328-336
We have previously developed a high-sensitive H2 gas sensor utilizing a high proton affinity of p-dipyridyldiketopyrrolopyrrole (p-DPPP). The sensor exhibits a remarkable reduction of the electrical resisitivity by two orders of magnitude under 0.05% H2 due to protonation at the para-site of the pyridyl ring. The present outstanding result motivated us to further investigate o- and m-derivatives in order to achieve an even better performance. However, the performance of these isomers was extremely poor. For this reason, the present investigation has been carried out in order to clarify the mechanism of the poor sensitivity from the standpoint of the electron delocalization (i.e. electron conduction) within the molecule as well as the electron hopping from one molecule to another (i.e. structural problem). As for the electron delocalization in p-DPPP, the change in electron density at the para-site(due to e.g. protonation)is found to be well propagated throughout the molecule, while those at the o- and m-sites are ineffective. This explains why p-DPPP is much superior for H2 gas sensors to o- and m-DPPPs. Another support is also given by the structure analysis of o-, m-, and p-derivatives. The N atom of the pyridyl ring (that serves as the antenna for protonation) remains unbonded (i.e. free) in p-DPPP and is capable of accepting protons. On the other hand, the N atoms are totally blocked by the formation of NH…N hydrogen bonds in o- and m-DPPPs. The above molecular and crystallographic considerations lead us to conclude that p-DPPP is, by far, advantageous to H2 sensors over o- and m-DPPPs.
