Intramolecular Spin Alignment in Photomagnetic Molecular Devices: A Theoretical Study

Abstract

Ground‐ and excited‐state magnetic properties of recently characterized π‐conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg–Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach. As the capability to quantitatively assess intramolecular exchange coupling J in the excited state makes it possible to undertake rational optimization of photomagnetic systems, DFT was subsequently used to model new compounds exhibiting different connection schemes for their functional components (P, B, SC). We show in the present work that it is worthwhile considering the triplet state of anthracene, that is, P when promoted in its lowest photoexcited state, as a full magnetic site in the same capacity as the remote SCs. This framework allows us to accurately account for the interplay between transient (3An) and persistent (IN, OV) spin carriers, which magnetically couple according to a sole polarization mechanism essentially supported by phenyl connector(s). From our theoretical investigations of photoinduced spin alignment, some general rules are proposed and validated. Relying on the analysis of spin‐density maps, they allow us to predict the magnetic behavior of purely organic magnets in both the ground and the excited states. Finally, the notion of photomagnetic molecular devices (PMMDs) is derived and potential application towards molecular spintronics disclosed.

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…Thus, the intrinsic triplet quantum yield of the chromophore moiety by normal SO‐ISC is very low (a very low efficiency of ISC from S 1 to T 1 ). In addition, the signal intensity of the verdazyl radicals ( 2 p and 5 pp ) was much stronger than that of the iminonitroxide radicals ( 1 p and 4 pp ), although a larger energy gap between D and Q was predicted by theoretical calculations . The magnitude of the intramolecular ferromagnetic exchange interaction ( J= 123 cm −1 [calcd] and 117±20 cm −1 [estimated from exp.…”

“…In addition, the signal intensity of the verdazyl radicals ( 2 p and 5 pp ) was much stronger than that of the iminonitroxide radicals ( 1 p and 4 pp ), although a larger energy gap between D and Q was predicted by theoretical calculations . The magnitude of the intramolecular ferromagnetic exchange interaction ( J= 123 cm −1 [calcd] and 117±20 cm −1 [estimated from exp. ]) was much larger than that of the dipolar interaction.…”

General Simulation of Transient ESR and Continuous‐Wave ESR Spectra for High‐Spin States Using a Density Matrix Formalism

“…[1] The investigations of photoinduced spin alignment and photoexcited high-spin states are of fundamental importance to clarify the general features of photoexcited states as well as novel photomagnetic devices. [1,11] Several high-spin excited states arising from excited-triplet/radical pairs (s-bonded systems [12] and coordination complexes [13] ) were also reported in the last decade.In order to investigate time-dependent phenomena, it is indispensable to simulate the system's behaviour quantum-mechanically from first principles. We reported the photoexcited quartet (S = 3/2) and quintet (S = 2) high-spin states of stable p-radicals.…”

Conformationally gated photoinduced processes within photosensitizer–acceptor dyads based on ruthenium(II) and osmium(II) polypyridyl complexes with an appended pyridinium group

“…The fact that this behavior is observed in such a closely coupled assembly is indicative of one of the basic feature of triplet states, namely their locally excited states nature (LES; Fig. 2), moreover capable of exhibiting rather strong electronic coupling without getting denaturized [124].…”