Controlling pairing of π-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Abstract

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials.

Conformational control over π-conjugated electron pairing in 1D organic polymers

“…39,40 These pre-optimizations are followed by full optimizations (atomic coordinates and a unit cell parameter) using the PBE0 hybrid functional 32 and the same light NAO basis set. We note that the PBE0 functional has been previously shown 36 to properly reproduce experimentally measured magnetic coupling coefficients of a synthesized circular p-TAM oligomer. 41 PBE and PBE0 optimizations are done employing a 6-1-1 and 18-1-1 G-centred Monkhorst Pack (MP) k-grid, respectively.…”

“…The electronic structure of p-TAM polymers is characterized by means of periodic DFT, using the hybrid PBE0 functional, 32 which was previously found to provide reliable electronic structure for TAM oligomers and 2D networks. [33][34][35][36] The calculations are done separately for the open-shell multi-radical and closed-shell quinoidal electronic solutions. The multi-radical solution is obtained from spin-unrestricted DFT calculations setting an anti-parallel spin initial guess on neighbouring aC centres.…”

“…9,21 On the contrary, TAM 2D-COFs where radical centres are further away from each other, such as the recently synthesized materials, [45][46][47] show a persistent multi-radical conguration that is insensitive to aryl ring attening. 36 Based on these ndings, here we consider ring-sharing p-TAM polymers (Fig. 1c) which may be understood as the 1D periodic analogues of Thiele's hydrocarbon (Fig.…”

“…This measure is typically used to characterize the open-shell/closedshell balance in extended TAM systems. [34][35][36] Fig. 2g shows hjm aC ji for the 1D material series (red curve).…”

Perspectives for polychlorinated trityl radicals

“…81 in an anisotropic manner such that some rings flatten and some twist more out-ofplane. 77,84,85 In hexagonal 2D CORFs this structural response can lead to pairing of electrons between neighbouring TAM centres and the transition from the open-shell multi-radical ground state to a diamagnetic closed-shell quinoidal state 86 (see Figure 5c).…”