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, magnetooptical switching, and "smart" materials. The magnetic properties of
these materials change when exposed to light, making them candidates for high-density optical data storage systems.
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.
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
Of the two, paramagnetism is the most interesting. Any molecule with at least one unpaired e- in it will be paramagnetic.
This is what the organic chemists call “free radicals”.
2+
NH3
H3C
H3C
N
CH3
CH3
H3N
Co
NH3
NH3
H3N
NH3
O
Eg*
Δo
T2g
The spin density can be visualized by semi-empirical calculations
such as UHF AM1
NH3
Hyperchem
H3C
H3C
N
O
CH3
CH3
H3N
Co
2+
NH3
NH3
H3N
NH3
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
N
C
+
N
PM3-semiempirical calculation of the spin density
Type of organic moieties with unpaired electrons
1. stable free radicals:
H3C
H3C
N
CH3
CH3
O
2. radical cations or anions:
N
+
N
C
3. organic diradicals with T groundstate:
3a. Carbenes or nitrenes, singlecentered (orthogonal orbitals)
3b. multi-centered radicals
(non-orthogonal orbitals)
H
F
F
H
N
R
H
H
C
H
C
H
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
# UnP e-’s
Mag. Moment
1
1.73
2
2.83
3
3.87
4
4.90
5
5.92
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
Eg*
Eg*
T2g
splitting
splitting
T2g
Similar things are observed in organic molecules:
Carbenes exist as Singlets or Triplets depending
on MO-splitting by a neighboring atom
Qualitative resonance
structure argument
F
CF2
F
F
F
F
F
-234.5 UHF T
CH2
F
-257.8 UHF S
H
H
H
H
-203.5 UHF T
F+
-161.9 UHF S
exp D = 9 kcal/mol
No such resonance
is possible
-
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
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
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
Y1(a) and Y1(b) formed by mixing Y4
Place
a-e-
in first NBMO
in and out of phase with Y1
Remember coefficients
in Y1 from SHMO
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.
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
Because coefficients are exchanged
and spins are exchanged
the same arguments hold
Thus we have a case of additive DSP
Let´s try to build a triplet
with similar arguments follows:
Competitive DSP: multiple unpaired e- polarize filled orbitals in the opposite way
no stabilization, thus
H
H
is in a S rather then a T ground state
H
H
A UHF-semi-empirical calculation (AM1) (using Hyperchem)
gives more quantitative results.
Hyperchem
AM1, UHF, S with geometry
optimization: -795.1kcal/mol
AM1, UHF, T with geometry
optimization: -779.2 kcal/mol
calculated spin density
-779
T
-795
S
remember: the SHMO-calc. Cannot discriminate S and T states
We just found cyclobutadien
H
H
H
H
to have an S groundstate
the HMO is very similar:
What´s about trimethylenemethane (TMM)
H
H
H
C
C
H
H
H
Does it has an S or T groundstate ?
SHMOprogram
DSP is additive for T
DSP is competitive for S
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.
Ferromagnetic coupler of a non-disjoint type structrure
active
non-active
H3C. +
non-disjoint
Approach to hyper-structured high spin molecules
Radical-centered high spin molecules.
Carbene-centered high-spin molecules
Magnetization curve of tetracarbene
Nitrogen-centered high spin molecule.
Spin-defects in dendrimeric structures may be tolerable
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.
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 ferromagnetic alignment of spins in a quasi-one-dimensional chain.
3. 2nd McConnell 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
Through space interactions
spatially fixed diphenylcarbenes. McConnell islets are formed in the
pseudoortho and pseudopara structures