reaching optimal light-induced intramolecular spin alignment within photomagnetic molecular device...
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Reaching Optimal Light-Induced Intramolecular Spin Alignment withinPhotomagnetic Molecular Device Prototypes
Ilaria Ciofini,*[a] Carlo Adamo,[a] Yoshio Teki,[b] Fabien Tuyras,[c] andPhilippe P. Lain*[c]
There is no better choice than light for interacting with mol-ecules. Light can be used 1) as a probe to address molecularentities; 2) as a trigger to manipulate molecular properties;
Abstract: Ground-state (GS) and excit-ed-state (ES) properties of novel pho-tomagnetic molecular devices(PMMDs) are investigated by means ofdensity functional theory. These organ-ic PMMDs undergo a ferromagneticalignment of their intramolecular spinsin the lowest ES. They are comprisedof: 1) an anthracene unit (An) as boththe photosensitizer (P) and a transientspin carrier (SC) in the triplet ES(3An*); 2) imino-nitroxyl (IN) or oxo-verdazyl (OV) stable radical(s) as thedangling SC(s); and 3) bridge(s) (B)connecting peripheral SC(s) to the Ancore at positions 9 and 10. Improvingthe efficiency of the PMMDs involvesstrengthening the ES intramolecularexchange coupling (JES) between tran-sient and persistent SCs, hence the
choice of 2-pyrimidinyl (pm) as B ele-ments to replace the original p-phenyl-ene (ph). Dissymmetry of the pm con-nectors leads to [SC-B-P-B-SC] regio-isomers int. and ext. , depending onwhether the pyrimidinic nitrogen atomspoint towards the An core or the pe-ripheral SCs, respectively. For the int.regio-isomers we show that the photo-induced spin alignment is significantlyimproved because the JES/kB value isincreased by a factor of more than twocompared with the ph-based analogue(JES/kB>+400 K). Most importantly,
we show that the optimal JES/kB value(+600 K) could be reached in theevent of an unexpected saddle-shapedstructural distortion of the lowest ES.Accounting for this intriguing behaviorrequires dissection of the combined ef-fects of 1) borderline intramolecularsteric hindrance about key Anpmlinkages, which translates into the flat-ness of the potential energy surface;2) spin density disruption due to thepresence of radicals; and 3) possibly in-tervening photochemistry, with Anacting as a light-triggered electrondonor while pm, IN, and OV behave aselectron acceptors. Finally, potentiali-ties attached to the [(SC)-pm-An-pm]intpattern are disclosed.
Keywords: density functional calcu-lations functional emergences magnetic properties molecularspintronics radicals
[a] Dr. I. Ciofini, Prof. Dr. C. AdamoLaboratoire dlectrochimie etChimie Analytique (CNRS UMR-7575)cole Nationale Suprieure de Chimie de Paris11, rue Pierre et Marie Curie, 75231 Paris Cedex 05 (France)Fax: (+33)144-276-750E-mail : email@example.com
[b] Prof. Dr. Y. TekiDepartment of Material ScienceGraduate School of ScienceOsaka City University3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 5588585 (Japan)
[c] Dr. F. Tuyras, Dr. P. P. LainLaboratoire de Chimie et Biochimie Pharmacologiques etToxicologiques (CNRS UMR-8601)Universit Paris Descartes, Facult de Mdecine45, rue des Saints Pres, 75270 Paris Cedex 06 (France)Fax: (+33)142-868-387E-mail : firstname.lastname@example.org
Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.200801405. It includes: Comput-ed geometrical parameters for Tdn, Ddn, Tdn* and Ddn* (n=58)and for HTd71 and HTd72 in both the GS and the ES (three tables);Computed highest SOMO for Dd5*, Td5* and Ref5* (in both axiallydistorted and saddle-shaped distorted conformations); Analysis ofbond-length alternation patterns in Dd5* (vs. Dd5) and Td5* (vs.Td5); Optimized structures of HTd71*, HTd72*, Td3bis*, Td4bis*and for Ref5 and Ref6 in both their mono-reduced and mono-oxi-dized states; Calculated spin-density patterns for Dd6 and Td6 (withcontour values of 5104 and 5105 a.u.) and for Td3bis* andTd4bis*; One figure showing pictorially the minor contribution ofresonance forms attached to ICTs within Ref5* in the lowest ES.
Chem. Eur. J. 2008, 14, 11385 11405 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 11385
3) as a quantum of energy to fuel the activity of molecularmachinery; and/or 4) as a quantum of information (input) tobe handled at the molecular level. Photochemical molecu-lar devices (PMDs), are typically light-driven moleculesdevised to act at their level for carrying out a specific task.They merit being referred to as supermolecules. The para-digm of PMDs comes from a combination of original para-digms of supramolecular chemistry and molecular (inor-ganic) photochemistry, which were adapted for the intramo-lecular medium and in particular for properties and func-tions relying on an electronic basis.[1b] These functional su-permolecules are often quite imposing in size as a result oftheir multicomponent nature. They are made up of weaklyinteracting components at the electronic level and are justlyreferred to as weakly coupled systems, the features of whichare accurately accounted for by a localized description.These supermolecules must be distinguished from large mol-ecules, which are conversely described within a delocalizedframework, featuring made-in-one-piece electronic behav-iors.[3,5, 6]
As part of our continuing research program to controland manipulate intramolecular electronic and magnetic cou-plings, we are interested in closely coupled (compact) multi-component systems. Our efforts are essentially directedtowards the related fields of artificial photosynthesis andmolecular electronics[7,12,13] (including molecular spintron-ACHTUNGTRENNUNGics[10c,14]). Inter alia, our work is aimed at incorporatingstrong and tunable intercomponent electronic and/or mag-netic couplings as full functional ingredients[10,11,1416] into anew type of PMDs, while retaining the hallmarks of super-molecules.[2,3,5] By skating on the edge of the paradigm
of PMDs, we presently investigate the borderline case ofclosely coupled and affiliated light-interacting systems,which may dramatically change their behavior (e.g., fromthat of supramolecular composite species, supermolecules,to that of made-in-one-piece large molecules)[7,9] upon slightalteration of the connection scheme of their functional com-ponents. As well as searching for new molecular photo-switching principles, the present study is aimed at both1) improving the photomagnetic functioning of a new typeof PMDs, namely photomagnetic molecular devices(PMMDs), and 2) gaining some basic knowledge relativeto the boundaries of multifunctional integration at the mo-lecular level.PMMDs are the PMDs that were recently adapted to sup-
port additional magnetic properties to perform operationsof potential interest in the nascent field of molecular spin-tronics.[10c,14, 18] These PMMDs are viewed as advanced mo-lecular-electronics devices, intended to rely on the spin ofthe electron rather than on its charge to handle informationat the molecular level. In this context, PMMD proto-types were recently designed by one of us (Figure 1) toundergo a photoinduced alignment of their intramolecularspins. These organic entities are systems of choice to studyclosely coupled molecular assemblies, because improvingtheir magnetic properties involves optimizing (here increas-ing) intercomponent electronic couplings. Indeed, an intrin-
sic feature of these PMMDs is the fairly strong intercompo-nent coupling they require for efficient mediation of (su-per)exchange magnetic interaction (J). For this specificreason, these molecular devices are potentially fully p-con-jugated systems. Of importance for the present work is therecognized sturdiness of the magnetic properties in thesense that, contrary to usual electronic features, magnetical-ly active components of molecular assemblies can stand(very) large electronic coupling and still fit in a localized de-scription reminiscent of that used for supermolecules in thefield of supramolecular photochemistry. Magnetism is,therefore, a means to trace the behavior of systems showingincreasing intercomponent electronic coupling up to the dis-appearance of the original electronic features of individualcomponents. It is also well established, however, that thepresence of a permanent source of paramagnetism (such asstable radicals) within a PMD assembly contributes to aslackening of some of the conventional selection rules andis therefore likely to disrupt its photochemical functioning,as we will show here.The multicomponent (polyad) systems herein considered
are made up of the following functional elements: 1) onephotosensitizer unit (P), namely anthracene (An), capableof photo-switching from its diamagnetic (closed-shell)ground state (GS) to its paramagnetic (e.g., open-shell trip-let radical) excited state (ES); 2) peripheral spin carriers
Figure 1. Schematic drawing together with labeling and numberingschemes of PMMDs (Ddn, Tdn) already investigated experimentally (n=1, 2) and theoretically (n=14).
www.chemeurj.org 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2008, 14, 11385 1140511386
(SCs), which are typically stable organic radicals like imino-nitroxyl (IN) and oxoverdazyl (OV); and 3) bridging ele-ments (B), which are connectors mediating the exchange in-teraction between the central P and the dangling SCs. Basi-cally, the working principle of such PMMDs is the following(Scheme 1). In the initial ground state, the peripheral SCs
are loosely antiferromagnetically coupled (JGS0) throughthe diamagnetic [B-P-B] assembly as the spin coupler. Uponlight excitation, P is promoted into its paramagnetic lowestexcited state (3P*), thus transiently behaving as a full SC inthe same capacity as other persistent ones. As a result ofenhanced and selective intersystem crossing, P* becomesferromagnetically coupled (JES>0