The Effects of Spin Polarization on Chemical Reactivity Dong Guo, J. Andrew Kouzelos, Larry D. Morris, Dani Arias-Rotondo and James K. McCusker*
Conservation of Spin in Energy Transfer
Research Goals
[M(pyacac)3{Re(bpy)(CO)3}3]3+
Our chemical systems are tailored to study the physical and photophysical properties of spin polarized
molecules. These molecules exhibit magnetic properties that allow us to study reactivity based on spin. In
addition to experiment, theoretical models can be adapted utilizing Heisenberg spin exchange. Heisenberg
spin exchange is a magnetic phenomenon in where a bridge for communication exists between unpaired
electron spins within a molecule and/or between molecules. Contained within this poster, spin polarization
manifests itself in studies of spin conservation in energy transfer processes, spin distribution effects on energy
transfer, synthetic modulation of spin distribution and creating spin-allowed transitions based on the
Heisenberg spin exchange model.
M = Cr3+, Co3+, or Ga3+
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0 20 40 60 80 100
S = 0
S = 1
S = 2
S = 3
S = 4
S = 5
Po
pu
lati
on
Dis
trib
uti
on
Temperature (K)
H = J S1 • S2
Spin Distribution Studies of MIIMII Energy Transfer Complexes
Photophysics of [M(pyacac)3{Re(bpy)(CO)3}3]3+
hv
Energy
Transfer?
300 350 400 450 500 550 600 6500
10
20
30
40
50
60
70
80
Wavelength (nm)
Mola
r A
bsorp
tivity (
X 1
03 M
-1cm
-1)
A
450 500 550 600 650 700 750 8000
20
40
60
80
100
120
Wavelength (nm)
Mo
lar
Absorp
tivity (
M-1
cm
-1)
Em
issio
n In
tensity
(norm
aliz
ed)
300 350 400 450 500 550 600 6500
10
20
30
40
50
60
70
80
Wavelength (nm)
Mola
r A
bsorp
tivity (
X 1
03 M
-1cm
-1)
B
450 500 550 600 650 700 750 8000
40
80
120
160
200
Wavelength (nm)
Mo
lar
Ab
so
rptivity (
M-1
cm
-1)
Em
issio
n In
ten
sity
(no
rma
lize
d)
A
0 500 1000 1500 2000 2500 3000 3500 4000
0
10
20
30
40
Em
issio
n I
nte
nsity (
mV
)
Time (ns)
0 1000 2000 3000 4000
0
10
20
30
40
50
Em
issio
n I
nte
nsity (
mV
)
Time (ns)
450 500 550 600 650 700 750 8000
1
Wavelength (nm)
Em
issio
n I
nte
nsity (
a.u
.)
B
Steady-State Absorption Spectra of MRe33+
Synthetic Control of Spin Distribution in Spin
Exchange Complexes
O O
C l
O
C l
O
M ' ( t r e ) ( t r e n ) M
1 M = G a 3 + M ' = G a 3 +
3 M = G a 3 + M ' = C r 3 +
5 M = C r 3 + M ' = C r 3 +
O O
C l
O
C l
O
M ' ( t r e n ) ( t r e n ) M
2 M = G a 3 + M ' = G a 3 +
4 M = G a 3 + M ' = C r 3 +
6 M = C r 3 + M ' = C r 3 +
[ M M ' ( t r e n ) 2 ( C A c a t , c a t ) ] 2 +
[ M M ' ( t r e n ) 2 ( C A s q , c a t ) ] 3 +
0 50 100 150 200 250 300 3500
1
2
3
4
5
6
Temperature (K)
Effe
ctive
Ma
gn
etic M
om
en
t
2
3
4
5
6
S = 1/2
S = 1
S = 3/2
two S = 3/2 CrIIIS = 5/2
17000 16000 15000 14000 13000 12000
0
1600 650 700 750 800
Ext
inct
ion
Coe
ffcie
nt
(x 1
03, M
-1cm
-1)
Energy (cm-1)
4
Wavelength (nm)
Cl
Cl
O
O
O
O
CrGa
N
N
NN
N
N
NN
3+
4
Extin
ctio
n C
oe
ffic
ien
t (X
10
3, M
-1cm
-1)
spin-allowed
2T1g
2Eg
4A2g E =4J
CrIII
(Oh)CrIII-CAcat,cat
(C2v)CrIII-CAsq,cat
(C2v)
a
b
Spin Exchange Effects in Tetraoxolene-Bridged
Bimetallic Complexes
35000 30000 25000 20000 15000
0
5
10
15
300 400 500 600 700 800
Ext
inct
ion
Coe
ffcie
nt (
x 10
3, M
-1cm
-1)
Energy (cm-1
)
Wavelength (nm)
1
3
18000 17000 16000 15000 14000 13000
0
1
Ext
inct
ion
Coe
ffcie
nt
(x 1
03, M
-1cm
-1)
Energy (cm-1)
4B2 → (4A1,4A2,
4B1)
Extinction C
oeff
icie
nt (X
10
3, M
-1cm
-1)
Extin
ctio
n C
oeffic
ient (
X 1
03, M
-1cm
-1)
Oh C2v
New Spin-Allowed Transitions
The photoexcited 3MLCT excited state of Ru(bpy)32+ is able to be quenched by 3O2 via energy transfer because out of the
range of possible spin allowed O2 excited states, the lowest singlet excited state of O2 is lower in energy than the 3MLCT
energy donor state.
In systems where electronic spin is a good quantum number, reactions
will occur when thermodynamically favorable and when the total spin
angular momentum space of the reactants has overlap with the spin
angular momentum space of the products.
N2 is unable to quench the 3MLCT because only triplet excited states of N2 are allowed if spin angular momentum is to be
conserved and the lowest energy triplet excited state of N2 is higher in energy than the donating excited state and thus the
reaction is not thermodynamically favored.
• Designed ReI-polypyridyl-transition metal assemblies
for testing the validity of spin selection rules in energy
transfer reactions.
• Cr3+, Co3+ and Ga3+ ions have similar charge-to-radius
ratios.
• Low spin CoIII (d6) and CrIII (d3) have different spin
states but similar energies in their ligand field
transitions.
• GaIII is spectroscopically silent due to its d10 electronic
configuration, which allows us to differentiate the
photo-physical properties of ReI-polypyridyls in the
absence of energy transfer.
Knight, T. E.; Guo, D.; Claude, J. P.; McCusker, J. K. Inorg. Chem. 2008, 47, 7249.
Plots of the electronic absorption spectra of
[Cr(pyacac)3{Re(bpy)(CO)3}3](OTf)3 (CrRe3, red trace) and
[Ga(pyacac)3{Re(bpy)(CO)3}3](OTf)3 (GaRe3, black trace).
Inset: Overlay of the emission spectrum of GaRe3 (purple
trace) with the electronic absorption spectrum of
Cr(phacac)3 (red trace); the latter was used as a surrogate
for the Cr(pyacac)3 core. Spectra were acquired in CH2Cl2
solutions at room temperature.
Plots of the electronic absorption spectra of
[Co(pyacac)3{Re(bpy)(CO)3}3](OTf)3 (CoRe3, red
trace) and GaRe3 (black trace). Inset: Overlay of the
emission spectrum of GaRe3 (purple trace) with the
electronic absorption spectrum of Co(phacac)3 (red
trace); the latter was used as a surrogate for the
Co(pyacac)3 core. All spectra were acquired in
CH2Cl2 solutions at room temperature.
Time correlated single-photon counting (TCSPC) data for CrRe3 at
= 580 nm following excitation at 375 nm. The red solid line
corresponds to a fit to a single-exponential decay model with Tobs =
4.8 ± 0.2 ns. Inset: Nanosecond time-resolved emission data for
GaRe3 at = 580 nm following excitation at 400 nm. The solid red
line corresponds to a fit to a single-exponential decay model with
Tobs = 630 ± 30 ns. All data were collected at room temperature in
deoxygenated CH2Cl2 solutions.
Nanosecond time-resolved emission data for CoRe3 at = 580 nm
following excitation at 400 nm. The solid red line corresponds to a fit to
a single-exponential decay model with Tobs = 640 ± 30 ns. Inset:
Steady-state emission spectra for CoRe3 (red trace) and GaRe3 (black
trace). The emission profile for the CoRe3 complex has been corrected
for the differential absorption of CoIII versus the Re-bpy moiety. All data
were acquired at room temperature in deoxygenated CH2Cl2 solutions.
• This class of covalently attached donor-acceptor systems allows us to study
how different spin states in the same molecule modulate energy transfer
reactivity by accessing different spin-allowed reaction paths.
• To correctly interpret the rates of energy transfer, the following complexes were
studied:
i. The ZnZn complex serves as our structural model to understand the
dynamics of the ruthenium 3MLCT excited state donor in the absence of
any energy transfer quenching due to MnII in the bimetallic acceptor.
ii. The MnZn complex allows us to investigate the intrinsic temperature
dependence of quenching via energy transfer to the MnII ion in the
absence of additional spin allowed pathways provided by Heisenberg
exchange.
iii. The MnMn complex allows us to observe the effects of the multiple spin-
allowed pathways made available via the Heisenberg spin exchange
between the ligand field states of both manganese centers in the
acceptor.
Soler, M.; McCusker, J. J. Am. Chem. Soc. 2008, 130, p.4708.
Soler, M.; McCusker, J. J. Am. Chem. Soc. 2008, 130, p.4708.
Spin Distribution in MII-MII Complexes
• The spin exchange interaction can be modeled by a
Heisenberg Hamiltonian.
• The MnMn complex exhibits small antiferromagnetic
coupling (J = 3 cm–1) which results in the two S = 5/2
MnII centers interacting to form an energetic ladder of
spin states ranging from ST = 0 to ST = 5.
• The entire spin ladder is energetically accessible below
100 K, allowing all spin dependent dynamics to be
probed in an optical glass.
• Population of the thermally accessible higher spin
states is controlled via temperature.
Correlation between the UB3LYP/6-311g**
calculated average α-spin density (NPA) at
the phenanthrosemiquinone (PSQ) oxygen
atoms and the J value in [Ni(tren)(3,6-di-R-
PSQ)]+ complexes.
The effective magnetic moment vs.
temperature for 3,6-di-H, -NH2 and -NO2
derivatives of [Ni(tren)(3,6-di-R-PSQ)]+. J
was established for the –NH2 derivative to
be -119 cm–1.
Turn on/off spin exchange by incorporating
CAsq,cat/CAcat,cat. Use (relatively) redox-inert metal
ions, leading to well-defined oxidation states of the
ligand. Incorporate GaIII to help differentiate physical
properties due to spin exchange.
Complexes 2 and 3 show temperature-
independent behaviors, indicating no spin
exchange in these molecules. The result of 5
reveals weak intra-molecular antiferromagnetic
coupling between two CrIII centers, whereas the
data of complexes 4 and 6 indicate strong spin
exchange coupling between CrIII and the radical
bridge.
When one GaIII ion was replaced by CrIII in the CAcat,cat bridged dimer, several new absorptions
appeared. Besides CAcat,cat- localized transition, the bands in mid-visible are assigned as charge-
transfer bands, whereas the lowest energy feature is assigned as spin-allowed d-d transitions of CrIII.
When CrIII is coupled with a semiquinone, the ligand-field states of CrIII are split and spin-allowed
transitions are created. The intensity of the bands associated with ligand-field states is enhanced. We
assign these two intense features as 2Eg derived state transitions (a and b), but further study by single-
crystal polarized absorption spectroscopy is needed.
Guo, D.; McCusker, J. K. Inorg. Chem. 2007, 46, 3257-74.
C l
C l
O
O
O
O
C r G a
N N
N N
N N
N N
2 +
3
In both compounds, the 3MLCT excited state has a spin multiplicity
of |SD*| = 1, which upon undergoing energy transfer to the
M(pyacac)3 core results in reformation of the singlet ground state of
the Re-bpy moiety. In the case of M = CrIII, the 4A2 ground state
creates a spin manifold in the reactant angular momentum space
spanning |SR| = 1/2, 3/2, and 5/2, which requires coupling to an
excited state of the acceptor having |SA| = 1/2, 3/2, or 5/2 in order to
realize a spin-allowed pathway. Angular momentum conservation is
clearly satisfied with the 4T2 excited state of the CrIII core (|SA*| = 3/2), as are thermodynamic considerations. Thus, dipolar energy
transfer can proceed through the commonality of S = 3/2 states in
both the reactants and products, and excited-state quenching of
the 3MLCT emission is observed. Upon replacement of CrIII by CoIII,
the thermodynamics of energy transfer are comparable; however,
the low-spin d6 configuration of the Co(pyacac)3 core fundamentally
alters the momentum conservation condition. Specifically, the 3MLCT 1A1 emission requires coupling to an excited state of the
CoIII having |SA*| = 1, not |SA*| = 0 which defines the 1A1 1T1
absorption. Dipolar energy transfer is therefore spin-forbidden for
the CoRe3 assembly, thus giving rise to emission from the Re-bpy
luminophore that is indistinguishable from the GaIII model complex.