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.

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

“…Photomagnetic molecular devices (PMMDs) [8] constitute the subclass of MSDs that function with light as the primary input stimulus. They potentially combine ISA with other types of intramolecular processes such as photoinduced ET and even electronic energy transfer (EnT); [64] these include specifics like enhanced and selective intersystem crossing (ISC).…”

“…[8,63] Thus, the magnetic site of OV is effectively made up of its four N atoms, whereas that of An + is essentially composed of up to the ten carbon atoms on the vertices, out of the 14 carbon atoms of the An backbone. Once again, features of the An + spin carrier closely resemble those of a 3 An* LES within AnOV*, [8a] and is accordingly worth noting as a magnetic "super" site.…”

Molecular engineering of liquid crystalline derivatives of 6-oxoverdazyl

“…40 The lower homologues 10a The role of the linking group L in the "arm" was briefly investigated using dodecyloxy derivatives 302 ( Figure 12). Thus, replacement of the COO group in 10a [12] with an azo group N=N in 11a [12] or removing it in 303 derivative 12a [12] resulted in elimination of the nematic phase and appearance of the tetragonal phase The effect of the "head" group at the C(3) position on mesogenic properties was investigated in the 311 series 10a [12]-10f [12] ( Figure 12). 41 Thus, replacing the CF 3 group in 10a [12] with another small polar group, 312 COOMe, in 10b [12] increased stability of the nematic phase by 11 K ( In order to compare effectiveness of the substituent at the C(3) position in mesophase formation,…”

“…virtual N-I transition temperatures for 10b [12]-10f [12] were extrapolated from 10 mol% binary mixtures with 324 the CF 3 derivative 10a [12]. Results in Table 4 show that the nematic phase stability follows the order: COOMe 10e [12] relative to C(3)-Ph derivative 10d [12] is presumably related to the polarity of the substituent.…”

“…Results in Table 4 show that the nematic phase stability follows the order: COOMe 10e [12] relative to C(3)-Ph derivative 10d [12] is presumably related to the polarity of the substituent.…”