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.