Intramolecular Spin Alignment within Mono-Oxidized and Photoexcited Anthracene-Based π Radicals as Prototypical Photomagnetic Molecular Devices: Relationships Between Electrochemical, Photophysical, and Photochemical Control Pathways

Abstract

AnOV is a pi-conjugated radical built from an anthracene (An) unit linked by a p-phenylene to an oxoverdazyl (OV) moiety. The mono-oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV(+) was demonstrated by combining spectroelectrochemistry, electron-spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations. The intramolecular spin alignment (ISA) within AnOV(+) results from the ferromagnetic coupling (J(electrochem)>0) of the two unpaired electrons located on the oxidized electron donor (An(+)) and on the pendant OV radical. The spin-density distribution pattern of AnOV(+) is akin to that of AnOV when photopromoted (AnOV*) to its high-spin (HS) lowest excited quartet (S=3/2) state. This high-spin state results from the ferromagnetic coupling (J(photophys)>0) of the triplet locally excited state of An ((3)An*) with the doublet ground state of OV. As a shared salient feature, AnOV(+) and AnOV* (HS) show a spin delocalization within the domain of activated An in either An(+) or (3)An* (nexus states) forms. The present study essentially contributes to establish and clarify relationships between electrochemical, photophysical, and photochemical pathways to achieve ISA processes within AnOV. In particular, we discuss the impact of the spin polarization of the unpaired electron of OV on electronic features of the An electron-donating subunit. Close analysis of this polarizing interplay allows one to derive a novel functional paradigm to manipulate electron spins at the intramolecular level with light and under an external magnetic field. Indeed, two original functional elements are identified: light-triggered donors of spin-polarized electrons and spin-selective electron acceptors, which are of potential interest for molecular spintronics.