Impact of short and long-range effects on the magnetic interactions in neutral organic radical-based materials

Abstract

The mutual influence of electronic structure and environment of the constituent units of neutral organic radical-based materials (radical dimers) is analysed by means of wave function calculations (Difference Dedicated Configuration Interaction, DDCI). Focus is put on the magnetic property modulations of two classes of neutral organic materials by inspecting both short- and long-range effects. The exchange coupling constant J for the high-temperature phase of the 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) material is calculated to be J= -95 cm(-1) at the DDCI level. The environmental electronic polarization is taken into account self-consistently using the individual polarizabilities of the atoms in a finite block of the crystal lattice (Discrete Reaction Field, DRF) and accounts for less than 5% of the calculated J value in TTTA. Furthermore, taking advantage of the chemical flexibility of the verdazyl radical family, the contribution of strong electron-withdrawing groups is analysed by extracting the J, U, t and K parameters from pairs of substituted verdazyl-based radicals. Our ab initio calculations of verdazyl radical pairs suggest that the addition of NO2 groups cause (i) the variations of the ferromagnetic and antiferromagnetic contributions to cancel out, leaving an almost constant exchange coupling constant, ca. J≈ 20 cm(-1), and that (ii) enhanced conduction properties can be anticipated. In contrast to inorganic analogues, one may conclude that the magnetic behaviour of neutral organic radical-based materials is mostly governed by the supramolecular arrangement, whereas environmental effects have a lesser impact.

Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions

“…Even at the electrostatic level, the Coulomb interactions between units are screened thanks to the environment. 37,38 In addition, the intrinsic polarizabilities of the building blocks are likely to give rise to stabilizing interactions. From previous investigations, and in the light of our current results on the [MV 2 ] 2(+) cation, the dispersion forces significantly overcome the Coulomb repulsion since a 45 kJ mol À1 binding interaction is calculated in vacuum (see previous section).…”

Bistability in Organic Magnetic Materials: A Comparative Study of the Key Differences between Hysteretic and Non‐hysteretic Spin Transitions in Dithiazolyl Radicals

“…However,i nh ysteretic spin-switch systems, the LT !HT transition takes place at ad ifferent temperature than that of the HT!LT transition. [23,24] Among the group of bistable DTAr adicals, the case of 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is especially remarkable, since its bistability region spans 70 degrees and encompassesr oom temperature (T c› = 310 K, T cfl = 230 K) [7,[25][26][27][28][29][30][31][32][33][34][35] (Figure 1a). Bistable materials based on transition-metal complexes have been associated with spin transitions for many years [3][4][5] ,b ut recent work has demonstrated that purely-organic radicals can also be versatile building blocks to achievebistability.…”

“…It is worth mentioning that the closely related family of dithiadiazolyl radicals has most recently furnished two examples of non‐hysteretic spin transition compounds . Among the group of bistable DTA radicals, the case of 1,3,5‐trithia‐2,4,6‐triazapentalenyl (TTTA) is especially remarkable, since its bistability region spans 70 degrees and encompasses room temperature ( T c↑ =310 K, T c↓ =230 K) (Figure a). The crystal structure of the 4‐cyanobenzo‐1,3,2‐dithiazolyl radical (4‐NCBDTA, hereafter referred to as 1 , see Figure b), in turn, is a prototypical example of a DTA‐based crystal featuring a non‐hysteretic spin transition .…”

Methods for describing open-shell systems: Following the trail of Rosa Caballol's research

“…Although the study of the magnetic coupling has long been (and probably still is) the major area of application, DDCI has also been used to study vertical excitations spectra, [64,65] ionization energies, and weakly avoided crossings, [66] potential energy surfaces in photochemistry, [67,68] relative stability of different biomimetic adducts, [54] hopping probabilities in doped systems or mixed valence compounds, [69][70][71][72] conduction properties of 1D organic chains, [73,74] and the electronic structure of systems with magnetoresistance effects, [75][76][77] among others.…”