Origin of Bistability in the Butyl-Substituted Spirobiphenalenyl-Based Neutral Radical Material
Summary (2 min read)
- The origin of bistability in the butyl-substituted spirobiphenalenyl-based neutral radical material Maria Fumanal¶, Juan J. Novoa, Jordi Ribas-Arino* Departament de Química Física and IQTCUB, Facultat de Química, Universitat de Barcelona, Av. Diagonal 645, 08028-Barcelona * firstname.lastname@example.org, email@example.com ¶Present address: Laboratoire de Chimie Quantique, Institut de Chimie UMR7177 CNRS-Université de Strasbourg, 1 Rue Blaise Pascal BP 296/R8, F-67007 Strasbourg, France.
- Therefore, the studies aimed at elucidating the origin of these barriers and at establishing the role of cooperative effects have the potential to offer most valuable hints on how to devise new bistable materials with improved properties.
- Below the spin transition temperature, the structures of the πdimers are governed by the potential energy surface (PES) of the ground singlet state (1Ag state), whose minimum structure features a partial localization of the unpaired electrons of each SBP radical in the superimposed phenalenyl (sup-PLY) rings, that is, on the phenalenyl (PLY) units directly involved in the π-dimer .
- Finally, the authors will decipher the mechanism of the coupling between the spin transition and the conformational rearrangement, they will demonstrate that the LT→HT phase transition is assisted by structural cooperative effects, and they will reveal that the hysteresis loop featured by 2 originates in the high-energy penalty associated with the conformational change of the butyl groups in the crystal lattice of the lowtemperature phase (subsection 4).
1) Phase transition of butyl-SBP: a spin transition coupled with a conformational
- The optimized structures of the LT and HT polymorphs were obtained by means of variable-cell geometry relaxations, in which the atomic positions and the lattice parameters are optimized simultaneously.
- A constant number of plane waves imply no Pulay stress but a decreasing precision of the calculation as the volume of the cell increases.
- The large cutoff employed in these calculations ensures that the artifacts arising from this change of precision are negligible.
- The starting atomic positions and initial lattice parameters for the relaxation of the LS(gau) and HS (anti) polymorphs were taken from the X-ray resolved structures of the LT and HT phases of 2 at 100 and 360 K, respectively.
- The optimizations of the isolated π-dimers of 2 (carried out with the goal of evaluating the gas-phase ΔEadia values) were also done with plane wave pseudopotential calculations using Vanderbilt ultrasoft pseudopotentials.
2) Origin of the main structural differences between the two polymorphs of 2.
- The analysis presented in this subsection was done using the results obtained in the previous subsection.
3) Driving forces of the phase transition of butyl-SBP. Order-disorder transition
- The vibrational entropy of the different polymorphs and the isolated π-dimers was evaluated after computing the vibrational frequencies of these systems in the harmonic approximation.
- The vibrational frequencies in the condensed phase were calculated by means of a finite-difference normal-mode analysis of the optimized structures.
- It thus follows that the computational strategy has been properly set up.
- All dynamic simulations were performed in the canonical ensemble using 23 the Nosé-Hoover chain thermostats100 in order to control the kinetic energy of the nuclei and the fictitious kinetic energy of the orbitals.
- In the simulations of the LT phase, the electronic structure of the π-dimers was that corresponding to their singlet ground state.
4) Origin of the hysteresis and the coupling between the spin transition and the
- Conformational change in the phase transition of butyl-SBP For the LS(gau) polymorph, in turn, five calculations were performed along the same rotation coordinate between the -107° and 0° values of the θ dihedral angle.
- To study the elementary steps of the LS(gau) à LS(anti) phase transition, the intermediate states connecting these two polymorphs (3gauche-1anti, 2gauche-2anti and 1gauche-3anti) were obtained by means of variable-cell geometry relaxations, in which the atomic positions and the lattice parameters are optimized simultaneously.
- Cell parameters of the reported LT-340 and HT-340 X-ray crystal structures of 2. Table S3.
- Cell parameters of the LT-0 minimum energy structure of butyl-SBP and of the LT crystallographic structures resolved at 100 and 340 K.
- The authors acknowledge the Spanish Government for financial support (Projects MAT201125972 and MAT2014-54025-P) and a “Ramón y Cajal” fellowship to J.R.-A. 2008, 7, 48-51. 41 X. Chi, M.E. Itkis, B.O. Patrick, T.M. Barclay, R.W. Reed, R.T. Oakley, A.W. Cordes, and R.C. Haddon, J. Am. Chem. Soc. 1999, 121, 10395-10402. 42 X. Chi, M.E. Itkis, K. Kirschbaum, A.A. Pinkerton, R.T. Oakley, A.W. Cordes and R.C. Haddon, J. Am. Chem. Soc. 2001, 123, 4041-4048. 43 X. Chi, M.E. Itkis, R.W. Reed, R.T. Oakley, A.W. Cordes and R.C. Haddon, J. Phys. Chem. B 2002, 106, 8278-8287. 44 X. Chi, M.E. Itkis, F.S Tham, R.T. Oakley, A.W. Cordes and R.C. Haddon, Int. J. Quant.
- 83 The ΔEadia value between both gauche-IN LS and HS minima is ca. 1 kcal/mol larger than the ΔEadia value between anti polymorphs .
- Therefore, the large increase in vibrational entropy associated with the conformational switch explains why the system undergo first the conformational switch even if the energy gap that is cleared in this transition is larger than that of the spin switch.
- 90 (a) J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865–3868 (b) J.P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 1997, 78, 1396. 91 S. Grimme, J. Comput.
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