an introduction into spin chemistry · 2015-12-31 · d.r. tyler laboratories: pushing back the...

D.R. Tyler Laboratories: Pushing Back the Forefronts of Science. 3

An Introduction into Spin Chemistry

Justin Barry

Group Meeting

12/16/2015


Basic Photochemistry

• Grotthus-Draper law

– Light must be absorbed

for a photochemical

reaction to take place

• Stark-Einstein Law

– Each photon of light

absorbed by a chemical

system, only one

molecule is activated for

a subsequent reaction

kF kIC

ε(S0→S1) ε(S0→T1)

kP kTS

kTS

Jablonski Diagram

(Does not attempt to

represent the molecular

shapes or sizes) Principles of Molecular Photochemistry: An Introduction, 1st edition.; University Science Books: Sausalito, Calif, 2008..


Potential Energy Diagrams

• Does better at depicting geometry

• Versions can also include vibrational probabilities (χv

2)

• T1 state is generally lower in energy than S1

• Only valid for same orbitals, ie. 1(π,π*) and 3(π,π*)

E

Internuclear Separation

kP

kTS

kF

S0

S1

T1

Principles of Molecular Photochemistry: An Introduction, 1st edition.; University Science Books: Sausalito, Calif, 2008..


Intersystem Crossing

• A radiationless

transformation

involving a transition

between two states

with different spin

multiplicity –IUPAC

• Also, technically

includes

phosphorescence

S0 S1 T1


A Vector Visualization of ISC

Pure S Pure T0 Mixed ST0 states

Spin rephasing

α

β

αα

ββ

αβ+βα

-1/2

+1/2 +1

-1

0

Principles of Molecular Photochemistry: An Introduction, 1st edition.; University Science Books: Sausalito, Calif, 2008..


Radical Pair Mechanism

• A and B may be part of the same molecule or closely spaced molecules

• Species C can be the signaling state

• Intersystem crossing will affect yield of C

• Intersystem crossing is affected by magnetic fields

Rodgers, C. T.; Hore, P. J. Proc. Natl. Acad. Sci. 2009, 106 (2), 353–360.


Energies and Chemical Reactions

• Thermal energy

– Room temp = 0.59 kcal•mol-1

• Light energy

– Green (500 nm) = 60 kcal•mol-1

• Typical heats of chemical reactions:

– 2-23 kcal•mol-1

• At 1 Tesla, splitting of energies

– 0.00267 kcal•mol-1

Rodgers, C. T. Pure Appl. Chem. 2009, 81 (1).


A Small Amount of Energy?

• “External magnetic fields affect the rate and extent of spin evolution, and hence products”

• How is this possible if the energy is far below kBT?

• Most of the energy supplied is in the excitation step, the signal man on the tracks merely guides -Rodgers


E


A Small Amount of Energy?

• “External magnetic fields affect the rate and extent of spin evolution, and hence products”

• How is this possible if the energy is far below kBT?

• Most of the energy supplied is in the excitation step, the signal (magnetic field) on the tracks merely guides -Rodgers


E


Magnetic Fields Lines and Migration

International Geomagnetic Reference Field 1990 (Blakely).


Experiments Using Birds

• Monochromatic light

Orientation of robins a) spectra from the light emitting

diodes used in the test b) orientation behavior under the

various lights.

Wiltschko, R.; Wiltschko, W. Biosensors 2014, 4 (3), 221–242.


Experiments Using Birds

• Using an applied magnetic field

a) Apparatus for testing birds b) orientation of robins

during spring migration in apparatus (left) applying a

magnetic field to divert magnetic north with measured

robin orientation (right) Emlen, S. T.; Emlen, J. T. The Auk 1966, 83 (3), 361–367. Wiltschko, W.; Gesson, M.; Wiltschko, R. Naturwissenschaften 2001, 88 (9), 387–390.


Theoretical Inclination Effects

• “Seeing” magnetic field lines

a) Proposed mechanism b) proposed receptor sensing of a magnetic

field line in the eye.

Emlen, S. T.; Emlen, J. T. The Auk 1966, 83 (3), 361–367. Wiltschko, W.; Gesson, M.; Wiltschko, R. Naturwissenschaften 2001, 88 (9), 387–390.


A Chemical Magnetoreceptor

• C-P-F triad

Maeda, K.; Henbest, K. B.; Cintolesi, F.; Kuprov, I.; Rodgers, C. T.; Liddell, P. A.; Gust, D.; Timmel, C. R.; Hore, P. J. Nature 2008, 453 (7193), 387–390



• C-P-F triad

Carotenoid




• C-P-F triad

Carotenoid

Porphyrin




• C-P-F triad

Carotenoid

Porphyrin

Fullerene



Action as a

Chemical

Compass

• Freeze sample in

nematic phase of a

liquid crystal

• Excitation at 532 nm

and observe at 950

nm absorption band

of C•+

• Blue trace is at 39 μT

and red 49 μT



Some Concluding Remarks

• Small magnetic fields can affect product distributions

• Magnetoreception: – Birds, turtles, mammals,

newts, magnetobacteria

• Hotly debated whether RPM is the biological receptor: – Magnetite model

– Electroreceptor model (salt water fish)

Němec, P.; Altmann, J.; Marhold, S.; Burda, H.; Oelschläger, H. H. A. Science 2001, 294 (5541), 366–368.

Lohmann, K. J.; Lohmann, C. Biol. Bull. 1993, 185 (1), 149–151.

Deutschlander, M. E.; Freake, M. J.; Borland, S. C.; Phillips, J. B.; Madden, R. C.; Anderson, L. E.; Wilson, B. W. Anim. Behav.

2003, 65 (4), 779–786

Arctic tern migration pattern

an introduction into spin chemistry · 2015-12-31 · d.r. tyler laboratories: pushing back the...

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