title: Reaching optimal light-induced intramolecular spin alignment within photomagnetic molecular device prototypes.
abstract: Ground-state (GS) and excited-state (ES) properties of novel photomagnetic molecular devices (PMMDs) are investigated by means of density functional theory. These organic PMMDs undergo a ferromagnetic alignment of their intramolecular spins in the lowest ES. They are comprised of: 1) an anthracene unit (An) as both the photosensitizer (P) and a transient spin carrier (SC) in the triplet ES ((3)An*); 2) imino-nitroxyl (IN) or oxoverdazyl (OV) stable radical(s) as the dangling SC(s); and 3) bridge(s) (B) connecting peripheral SC(s) to the An core at positions 9 and 10. Improving the efficiency of the PMMDs involves strengthening the ES intramolecular exchange coupling (J(ES)) between transient and persistent SCs, hence the choice of 2-pyrimidinyl (pm) as B elements to replace the original p-phenylene (ph). Dissymmetry of the pm connectors leads to [SC-B-P-B-SC] regio-isomers int. and ext., depending on whether the pyrimidinic nitrogen atoms point towards the An core or the peripheral SCs, respectively. For the int. regio-isomers we show that the photoinduced spin alignment is significantly improved because the J(ES)/k(B) value is increased by a factor of more than two compared with the ph-based analogue (J(ES)/k(B)>+400 K). Most importantly, we show that the optimal J(ES)/k(B) value ( approximately +600 K) could be reached in the event of an unexpected saddle-shaped structural distortion of the lowest ES. Accounting for this intriguing behavior requires dissection of the combined effects of 1) borderline intramolecular steric hindrance about key An-pm linkages, which translates into the flatness of the potential energy surface; 2) spin density disruption due to the presence of radicals; and 3) possibly intervening photochemistry, with An acting as a light-triggered electron donor while pm, IN, and OV behave as electron acceptors. Finally, potentialities attached to the [(SC)-pm-An-pm](int) pattern are disclosed.
authors.affiliation: Laboratoire d'Electrochimie et Chimie Analytique (CNRS UMR-7575), Ecole Nationale Supérieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75231 Paris Cedex 05, France.
authors.forename: Ilaria
authors.initials: I
authors.lastname: Ciofini
authors.forename: Carlo
authors.initials: C
authors.lastname: Adamo
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Teki
authors.forename: Fabien
authors.initials: F
authors.lastname: Tuyèras
authors.forename: Philippe P
authors.initials: PP
authors.lastname: Lainé
mesh_terms: D000975:Antioxidants / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D008280:Magnetics; D008956:Models, Chemical; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry; D010777:Photochemistry; D011789:Quantum Theory
chemical_list: D000975:Antioxidants; D005609:Free Radicals; D009589:Nitrogen Oxides; C039900:nitroxyl