Origin of Bistability in the Butyl‐Substituted Spirobiphenalenyl‐Based Neutral Radical Material

Abstract

One of the most remarkable bistable materials reported so far is made of π dimers of a butyl-substituted spirobiphenalenyl boron radical (butyl-SBP). The phase transition of this material, which is accompanied by changes in its optical, conductive, and magnetic properties, occurs with a hysteretic loop 25 K wide centered at 335 K. Herein, a computational study is presented aimed at deciphering the origin of this hysteresis. The phase transition of butyl-SBP consists of a spin transition of the constituent π dimers coupled with an order-disorder transition involving the butyl chains linked to the nitrogen atoms of the superimposed phenalenyl rings of the π dimer. Below 335 K, the terminal methyl group of the butyl chains adopts a gauche conformation with respect to the methylene unit bonded to the nitrogen atom. Above 335 K, the methyl group is in an anti conformation and exhibits dynamic disorder. The gauche→anti conformational rearrangement triggers the spin transition of the π dimers and is responsible for the hysteretic behavior of butyl-SBP. Specifically, the onset of the phase transition in the heating mode, and thus, the width of the hysteresis loop, are governed by the high energy cost and strong structural cooperative effects associated with this conformational change. Our results show that coupling a spin switch with a conformational switch in a molecular crystal provides a promising strategy in the design of new bistable materials.

Two different mechanisms of stabilization of regular π-stacks of radicals in switchable dithiazolyl-based materials

“…As a result, these two crystals have become notable examples of magnetic bistability, a technologically relevant property that is sought--after 53,54 due to the memory effect intrinsically associated with a hysteresis loop. 55 Other organic radicals exhibiting magnetic bistability include additional 1,3,2-dithiazolyls 33,56 , bis--1,2,3--dithiazolyls 57,58 , nitroxides 59 , spirobiphenalenyls 60,61 , and dithiadiazolyls. 62,63 The hysteresis loop of 1 is centered around 323 K and spans 46 K (see Figure 1, right).…”

Effect of Substitution on the Hysteretic Phase Transition in a Bistable Phenalenyl‐Based Neutral Radical Molecular Conductor

“…The presence of the hysteresis and bistable physical properties of undoped [Bu] 2 B molecular crystals were originally ascribed to the existence of the energy barrier for the transition between LT and HT polymorphs . Recently, the nature of this energy barrier was investigated in a computational study by the Ribas‐Arino group and related to the gauche to anti conformational rearrangement of the butyl groups attached to nitrogen atoms on both sides of the boron atom accompanied by a significant expansion of the crystal in HT phase, followed by a change of magnetic state from LS to HS. The study found that out of four possible polymorphs, LS (gauche), LS (anti), HS (gauche), and HS (anti), only LS (gauche) and HS (anti) are thermodynamically stable at low and high temperatures, respectively.…”

“…In some of the warming‐cooling cycles, the entire conductivity change occurs instantaneously in one step, manifesting the intrinsic first‐order character of the phase transition. It has been suggested that the conformational changes involve cooperative effects when the rearrangement of one butyl group can reduce the potential barriers for rearrangement of the neighboring butyl groups leading to the avalanche type spreading of the conformational changes across the whole crystal …”

“…At the phase transition, the electrons move predominantly to the non‐overlapped non‐interacting section of the dimer thus entering a high spin (HS) triplet state, which can be observed in magnetic studies as an increase in the spin per molecule. This transition is accompanied by a large change in the crystal cell volume, and there is a large energy barrier between the two states that causes the high temperature of the phase transition and its hysteretic behavior . However, throughout the phase transition the monoclinic P 2 1 / c space group crystal structure remains intact .…”

“…It is one of the few molecular compounds with bistability in the vicinity of room temperature . The nature of the phase transition from the low‐temperature low‐spin state to a high‐temperature high‐spin state has been extensively studied both experimentally and computationally, and has been associated with the change of location of the unpaired spins within a [Bu] 2 B dimer . At low temperatures (LT), the spins predominantly occupy an overlapped section of the dimer which causes the spins to pair antiferromagnetically into a low‐spin (LS) singlet state (Figure ).…”