Very Strongly Ferromagnetically Coupled Diradicals from Mixed Radical Centers: Nitronyl Nitroxide Coupled to Oxoverdazyl via Polyene Spacers

Abstract

We predict extremely large and positive intramolecular magnetic exchange coupling constants (J) for coupled diradicals constructed from nitronyl nitroxide (NN) and oxoverdazyl (o-VER). These radicals have the general formula o-VER(N)-nC-NN where nC represents an olefinic spacer with n = 0, 2, 4, 6, and 8. Species like o-VER(C)-nC-NN have negative coupling constants. The atoms in the parentheses show the point of attachment of the coupler to the verdazyl moiety. Both the N-linked series and C-linked series have comparable stability. The triplet molecular geometries were optimized by the density functional (UB3LYP) method using the 6-311 g(d,p) basis set. This was followed by single-point UB3LYP calculations using 6-311++g(3df,3pd) basis. To calculate J, single-point broken-symmetry computations were performed on the optimized triplet geometries and using the same basis set. The N-linked diradicals coupled through conjugated polyenes are topologically different. These are found to have coupling constants of the order of 1000 cm(-1), whereas the C-linked diradicals show coupling constants of the order of -100 cm(-1). In general, for both cases, the absolute magnitude of the coupling constant decreases with the increase in the length of the spacer.

Intriguing strain-governed magnetic phase transitions in 2D vanadium porphyrin sheets

“…As shown in Table S1 (ESI†), the bond lengths (V–V and V–N) increase with increasing tensile strain, which implies that expansion of the sheet increases the coupling distance between the spin centers and thus could weaken the magnetic spin couplings among the spin centers. 39 In the 2D V-PP4 sheet, V atoms act as spin sources which undergo the magnetic spin coupling through the porphyrin framework. As the distance between two adjacent V atoms increases (from 8.402 to 9.075 Å) by tensile strain, the intensity of the magnetic coupling is remarkably weakened, as shown in Fig.…”

Quantum chemical approaches for controlling and evaluating intramolecular magnetic interactions in organic diradicals

“…The strength of the magnetic coupling between radicals generally decreases as coupler length increases. [23,31] Therefore, proper classification of the coupler and evaluation of its capability to generate a strong magnetic interaction is advantageous for the tailored design of OMMs. In this section, the classification and systematic evaluation of the magnetic strengths of coupler units will be discussed.…”

Verdazyls and Related Radicals Containing the Hydrazyl [R2N‐NR] Group

“…Even the 6-oxoverdazyl diradical, in which the two radicals are directly linked, remains an open shell singlet diradical in its ground state, with the triplet excited state some 760 cm −1 higher in energy. 52 Electronic and magnetic communication has been investigated in polyradicals of general structure 34, in which a verdazyl is linked by a π -conjugated spacer "X" to a different open shell chromophore, for example, nitronyl nitroxide, 82 tetrathiafulvalene radical cation, 81 triplet nitrene (generated by photolysis of the corresponding azide), 63,85 or triplet excited states of organic compounds, such as pyrene or anthracene. 74,86 In all of these cases, where spin-spin interactions were probed quantitatively (using EPR spectroscopy and/or computational studies) the exchange coupling energies remain on the order of a few hundred wavenumbers.…”