Getting closer to the intrinsic properties of Ni2+salen polymer semiconductors accessed by chain isolation inside silica nanochannels

Abstract

The scientific problem aimed to be solved by our research is to improve the charge transport of Ni²⁺salen polymer semiconductors by isolation of individual chains inside mesoporous silica channels capable of ensuring the suppression of interchain interactions that cause the charge carrier trapping in the continuous polymer films. Elimination of charge trapping is the primary motivation for energy transport improvement in polymer semiconductors serving as interconnections in molecular electronics. With this in mind, we prepared molecular wires based on structurally privileged ortho substituted Ni²⁺salens, offering the molecular structure facilitating its linear electropolymerization growth inside the confined space of a silica matrix containing vertical channels that are 2 nm in diameter. We noticed that the analogical embedment of ortho unsubstituted poly(Ni²⁺salen)s was not possible. Thus, we used quantum chemical calculations utilizing density functional theory to explain this phenomenon. Hence, we succeeded in recognizing structure–property relationships of Ni²⁺salens governing their local electropolymerization ability inside vertical nanochannels. Isolation of polymer chains allowed us to gain insight into the intrinsic properties of Ni²⁺salen polymers creating a better understanding of their charge transport and polymerization mechanisms. The prepared molecular wires exhibited improved electronic properties evidenced by electrochemical measurements revealing the energetically favored charge transporting polymer forms and the increased ratio between anodic and cathodic charges. Furthermore, we directly evidenced the presence of a Ni²⁺salen polymer inside silica nanochannels using high-resolution X-ray photoelectron spectroscopy combined with very low-energy argon ion beam sputtering, as well as using atomic force microscopy and transmission electron microscopy.