Probing Electronic Communication in Stable Benzene-Bridged Verdazyl Diradicals

Abstract

Two benzene-bridged N,N'-bis(isopropyl)6-oxoverdazyl diradicals 7a (1,4-benzene-bridged) and 7b (1,3-benzene-bridged) were prepared and studied by an array of physicochemical techniques aimed at elucidating the intramolecular electronic and magnetic coupling between verdazyl chromophores. The very high stability of these diradicals permits comprehensive investigations of their properties for the first time. The UV-vis spectra suggest negligible direct conjugative overlap involving the radical SOMOs, although significant differences in higher energy absorptions suggest that radical orbitals other than the SOMO are likely communicating via the central p-phenylene bridge in 7a. The electrochemical features of the two diradicals are nearly identical; in each derivative, both radicals are oxidized essentially independently while the reductions occur in a stepwise manner with differences of approximately 100 mV between the two reductions. EPR and magnetic susceptibility collectively indicate that the para-bridged diradical is weakly antiferromagnetically coupled while the meta analogue is weakly ferromagnetically coupled, in accord with the topology of the substitution pattern on the central benzene ring.

Synthesis and characterization of conjugated/cross-conjugated benzene-bridged boron difluoride formazanate dimers

“…Closely related benzene-bridged diradicals p-12 and m-12 have been shown to undergo reversible electrochemical oxidation and reduction to their closed shell bis-cation and bis-anion forms. 21 In these systems, electrochemical reduction occurred in two well-resolved steps, which supports the interpretation of our results.…”

Magnetic Phase Transition in the Verdazyl Biradical Crystalp-BIP-V2

“…. Several benzene-bridged verdazyl biradical compounds are estimated to have weak intramolecular interactions, whose absolute values are less than 50 K [7]. Since the estimated value of J AF is quite large compared with those in them, we can expect an existence of strong or multidimensional intermolecular AF interactions.…”

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

“…Since the types and strengths of the magnetic interactions are sensitive to the magnetic centers and the topological combinations of the centers through coupler units, [23,[29][30][31][32][33][34] a fundamental understanding of magnetic couplings according to the spin sources (radicals) and couplers is essential for the design of high-spin OMMs possessing strong magnetic interactions. As one of the earliest prediction of high-spin molecules, Hoffmann proposed that when the energy difference between the two singly occupied molecular orbitals (SOMOs) of a diradical is small enough (less than about 1.5 eV), the two orbitals are almost degenerate and thus have parallel spins to serve Hund's rule.…”

“…The rule of spin alternation in the UHF or UDFT levels predicts the magnetic ground states without fail, although counter examples against the former regarding the SOMO interactions were reported. [23] In addition, it was known that intramolecular magnetic interactions between spin sources through a coupler can be influenced by several other factors: the geometrical connectivity, [23,[29][30][31][32][33][34] coupler chain length, [40][41][42][43] aromatic nature of a coupler, [32] dihedral angle, [44,45] and so on.…”