Organic-Based Magnets

2. Organic-Based Magnets In 1986, Miller and Epstein discovered the first organic material to become magnetically ordered at -268 degrees 1991 discovery of the first organic material to exhibit magnetism above room temperature Impact: Organic magnets are lighter, more flexible, and less energy intensive to make than conventional metal and ceramic magnets. Applications now being pursued include magnetic shielding, magneto- optical switching, and "smart" materials. The magnetic properties of these materials change when exposed to light, making them candi- dates for high-density optical data storage systems.

3. Nebraska Chemists Create First Plastic Magnets Lincoln - Nov. 25, 2001 "There are already known organic magnets, but they are based on crystals of small molecules," "What is unique about this research is this is the first organic polymer that can be said to be magnetic." S. Rajca et al.

4. Fundamentally, magnetism is a phenomenon of moving electric charges. Move a stream of electrons in a circle … … or make a charged particle spin and you have magnetism. For our purposes, we’ll talk about two classes of magnetism: dimagnetism: a substance where all electrons are paired giving no overall magnetic field paramagnetism: a substance with at least one unpaired electron, giving an overall magnetic field to the molecule

5. 2+ Eg* Δo T2g Of the two, paramagnetism is the most interesting. Any mole- cule with at least one unpaired e- in it will be paramagnetic. This is what the organic chemists call “free radicals”.

6. The spin density can be visualized by semi-empirical calculations such as UHF AM1 2+ Hyper- chem

7. Odd e- species often have localized spin densities, but a single e- can also be delocalized over many centers, as in the viologen cation radical PM3-semiempirical calculation of the spin density

8. Type of organic moieties with unpaired electrons 1. stable free radicals: 2. radical cations or anions: 3. organic diradicals with T groundstate: 3a. Carbenes or nitrenes, single- centered (orthogonal orbitals) 3b. multi-centered radicals (non-orthogonal orbitals) F F

9. The amount of paramagnetism is directly related to the number of unpaired e-: u2 = 4s(s+1) where s = unpaired electron spin (1/2 for each unpaired e-) One unpaired e-, s = ½, and u2 = 4(1/2)(1/2 + 1) = 3, u = 1.73 BM

10. # UnP e-’s Mag. Moment 1 1.73 2 2.83 3 3.87 4 4.90 5 5.92 Eg* splitting Eg* splitting T2g T2g Co(H2O)6+2 has a measured magnetic moment of 3.9 Co(NH3)6+2 has a magnetic moment of 1.7. Co(H2O)6+2 is a high spin complex Ep > splitting Co(NH3)6+2 is a low spin complex Ep < splitting

11. Qualitative resonance structure argument + - -234.5 UHF T -257.8 UHF S No such resonance is possible -161.9 UHF S exp D = 9 kcal/mol -203.5 UHF T Similar things are observed in organic molecules: Carbenes exist as Singlets or Triplets depending on MO-splitting by a neighboring atom F F CF2 F F H H CH2 H H

12. Spin Polarization Dynamic or Double Spin Polaization (DSP) helps understanding e- - e- repulsion Model requires different orbitals for different spins (a and b) Additive DSP: multiple unpaired e- polarize filled orbitals in the same way Competitive DSP: multiple unpaired e- polarize filled orbitals in the opposite way Java-Program SHMO

13. Place a-e- in first NBMO Check for Additive or competitive DSP Additive DSP: multiple unpaired e- polarize filled orbitals in the same way Competitive DSP: multiple unpaired e- polarize filled orbitals in the opposite way

14. Y1(a) and Y1(b) formed by mixing Y4 in and out of phase with Y1 Place a-e- in first NBMO Remember coefficients in Y1 from SHMO Check for additive or competitive DSP Additive DSP: multiple unpaired e- polarize filled orbitals in the same way Competitive DSP: multiple unpaired e- polarize filled orbitals in the opposite way This polarization reduces coulombic interaction between Y1(b) and NBMO(a), and enhances exchange repulsions between Y1(a) and NBMO(a). Both reduce the total energy.

15. Because coefficients are exchanged and spins are exchanged the same arguments hold Thus we have a case of additive DSP Check for Additive or competitive DSP Additive DSP: multiple unpaired e- polarize filled orbitals in the same way Competitive DSP: multiple unpaired e- polarize filled orbitals in the opposite way

16. with similar arguments follows: Competitive DSP: multiple unpaired e- polarize filled orbitals in the opposite way no stabilization, thus is in a S rather then a T ground state Let´s try to build a triplet

17. calculated spin density AM1, UHF, S with geometry optimization: -795.1kcal/mol T -779 AM1, UHF, T with geometry optimization: -779.2 kcal/mol S -795 A UHF-semi-empirical calculation (AM1) (using Hyperchem) gives more quantitative results. Hyperchem

18. remember: the SHMO-calc. Cannot discri- minate S and T states We just found cyclobutadien to have an S groundstate the HMO is very similar: What´s about trimethylenemethane (TMM) Does it has an S or T groundstate ?

19. SHMO- program T is more stable DSP is additive for T DSP is competitive for S

20. Disjoint and Non-Disjoint Classification of Diradicals Nomenclature based on connectivity pattern of two moieties of a diradical. If diradicals are composed of active and inactive sites of radical moieties, they are classified as non-disjoint. Active means non-zero spin density, non-active means zeo spin-density.

21. non-active active H3C. + non-disjoint Ferromagnetic coupler of a non-disjoint type structrure

22. Approach to hyper-structured high spin molecules

23. Radical-centered high spin molecules.

24. Carbene-centered high-spin molecules Magnetization curve of tetracarbene

25. Nitrogen-centered high spin molecule.

26. Spin-defects in dendrimeric structures may be tolerable

29. Towards organic magnetic materials 1. Mataga Mechanism hypothetical high-spin plane two-dimensional structures, composed from triplet diphenylcarbenes integrated in an entire conjugate system (Mataga mechanism). Shown: Similar systems with triphenylmethane radicals was considered as the potential predecessors of organic ferromagnetic polymers.

30. 2. ion-radical salts of the type ...D+· A-· D+· A-· ... constructed as quasi-one-dimensional spin chains, in which the cation-radicals of the donor D+· are strictly alternated with the anion-radicals of the acceptor A-· (the first McConnell mechanism). It was supposed that virtual excited states of the pairs D+· A-· can occur, which stabilize its high-spin triplet state and ensure ferro- magnetic alignment of spins in a quasi-one-dimensional chain.

31. 3. 2ndMcConnell mechanism: For spin propagation in aromatic radicalsspin delocalization and spin polarization are important. They lead to situations where local spin is oriented oppositely to the total spin of the unpaired electron. If in a crystal radicals are arranged in such a way that positive spin density regions of one radical are situated most closely to the negative spin density regions of another radical (so called islets are formed). In such a system, antiferromagnetic interatomic exchange at interradical contact sites will create two spin subsystems: positive spin densities will be oriented in one direction, while negative spin densities - in the opposite one (as in a ferrimagnetic substance). But positive spin densities far exceed negative ones, and the total spin will correspond to a ferromagnetic ordering Symmetric (I) and asymmetrical (II) structures of the pair of allyl radicals. McConnell islets are present in structure II and absent in structure I. Schematic overlapping of pi*-orbitals, containing unpaired electrons

32. Through space interactions spatially fixed diphenylcarbenes. McConnell islets are formed in the pseudoortho and pseudopara structures