Quantum engineering of mixed-valence 1D conjugated polymers

One dimensional conjugated polymers (1DCPs) that host periodic arrays of unpaired electrons are gaining increasing attention as atomically-precise correlated materials for future quantum technologies. 1DCPs based on triarylmethyls (TAM) are particularly interesting, due to the persistent nature of TAM radicals and the possibility to control the delocalization of their unpaired electrons via different means. However, the known strategies to effectively control the quantum ground-state of these organic systems is still limited. Here, by means of first principles density functional theory (DFT) calculations, we propose the use of a rational periodic substitution of radical sp2 carbon (C) sites in TAM 1DCPs by sp2 nitrogen (N) atoms, as a means to tailor the quantum state of the resulting mixed-valence (mv) 1DCP. In particular, we explore a fully alternating N-substitution pattern (NCNC) and a semi-alternating one (NNCC), and show that the former gives rise to a robust multiradical open-shell configuration, whereas the latter leads to a closed-shell quinoidal state. Via ab initio molecular dynamics simulations, we demonstrate that such quantum engineering of mv-1DCPs is robust to thermal fluctuations at room temperature, which highlights the technological viability of our approach for future molecular scale quantum electronics and spintronics.