Electronic Studies of High-Spin Organic Molecules: From the Effects of Substituents on Exchange Coupling to New Heterospin Species

Abstract

Design, synthesis, and characterization of novel high-spin species are critical in better understanding the field of molecular magnetism. It is also critical in advancing the field to a level of application, where new magnetic materials can be used in daily life. In this work, structure-property relationships of bis(semiquinone)s (SQ's) were studied, substituent effects on exchange coupling were investigated, and a series of novel, heterospin, complexes were synthesized and characterized.

A strong comparison of exchange coupling to mixed valency is made in a bis(SQ) based on magnetic and electrochemistry data. This comparison is possible because of the isostructural species involved in the study, a Trimethylenemethane-Type (TMM) coupled bis(SQ), and a similarly mediated mixed-valent species. The sole difference between the two species being a single electron.

A substituent effect was found to operate in an isostructural series of three meta-phenylene-Type (MPH), ferromagnetically coupled bis(SQ)s. Using common electron donating, withdrawing, and 'neutral' substituents, (NMe₂, NO₂, and t-Bu respectively) it was shown that both donating and withdrawing groups attenuate the singlet-triplet gap in the ground-state triplet species. A relatively simple HÜckle molecular orbital explanation describes the effect. It is the first time that a singlet-triplet gap of a ground-state triplet biradical has been affected by substituents.

Finally, a series of new heterospin complexes consisting of a nitronyl nitroxide (NN), semiquinone (SQ), and metal (II) ion were synthesized and characterized. These species are among the limited few examples of a molecular species containing up to three entirely different spin-carriers. Also, of great importance, is the novel variation on the TMM-type coupling within the complexes. This ferromagnet coupling between the nitronyl nitroxide and the semiquinone portions of the complex is so strong (lower limit of + 310 cm⁻¹) that even at room temperature, only the triplet ground-state is populated, while no appreciable population of the lowest lying singlet state can be detected.