David L. Cedeno, Ph.D.Department of ChemistryIllinois State UniversityBox 4160Normal, Illinois, 61790-4160

Ph: +1-309-438-5595E-mail: dcedeno@ilstu.edu

Laser-based Methods Applied to the Study of Metal-Olefin Interactions and the

Photophysics of Sensitizers

General: Use spectroscopic and computational tools to study thermodynamic and kinetic aspects of different systems

Research Focus

1. Energetics and kinetics in Organometallic Chemistry. Relevance: Agriculture: Rational design of anti-ripening compounds

2. Effects of molecular structure on yields of triplet state of oxygen photosensitizers. Relevance: Clinical: Rational design of efficient photodynamic therapy compounds and fluorescent diagnostic probes.

Why are we interested on metal-olefin bonding interactions?

Metal-olefin complexes are involved in biological systems: growth of plants, senescence of flowers, ripening of fruits.

We want to understand how olefins bind to metals: why do some olefins bind better than others to a

particular metal? why do some metals bind better than others to a

given olefin? How can we control metal-olefin interactions?

Lots of processes involve breaking and forming metal-olefin bonds.

Metal-olefin complexes are involved in many catalytic systems: polymerization, hydrogenation, epoxidation, etc.

Ethylene as a plant hormone

•Ethylene binds to protein receptors in plants to signal developmental processes as seed germination, plant growth, fruit ripening, flower abscission and senescence.• Ethylene signaling involves a family of sensor/response regulator proteins (ETR, EIN, AIN, etc. The receptor is a negative regulator of a protein kinase.

Ciardi and Klee, Annals Bot., 2001, 88, 813Chang and Stadler, BioEssays, 2001, 23, 619

Anti-ripening control: Blocking ethylene action

•It has been proposed that ethylene action at the receptor site requires two steps:

•Competitive antagonists block the receptor by bonding to it for a longer time that ethylene does, thus preventing the activation of the receptor for signaling.

Cu +L

CuL

CuL

+

inactive inactive active

Cu +L

CuL

CuL

+

inactiveinactive

active

very slow

Burg and Burg, Science, 1965, 148, 1190 Sisler and Serek, Bot. Bull. Acad. Sin. 1999, 40, 1

Anti-ripening control: Blocking ethylene action

•Known competitive antagonists include:

N N

UV light

?

•An understanding of the metal-olefin interaction is important in designing anti-ripening compounds.•Why are cyclic olefins so special? Ring strain affects olefin-receptor interaction. •Are there any other effects? Is it possible to control the strength of the metal-olefin bond?

Towards a quantitaive description of metal-olefin interactions: Extending the Dewar-Chatt-Duncanson Model

Dewar, Bull. Chem. Soc. Fr., 1951, 18, C71-79; Chatt and Duncanson, J. Chem. Soc., 1953, 2939)

bond

bond

Qualitative nature of model prevents any complete rationalization of metal-olefin bond strengths, because it does not take into account all the factors involved in the interaction. We propose a quantitative extension to DCD.

Gather more experimental data: Measurement of Bond Enthalpies

Bond energies reflect the strength of the interaction

(L)nM-olefin + heat (L)nM + olefin

Use Quantum Mechanical Calculations to account for all factors in the interaction: Bond Energy Decomposition

Experimental Techniques: Time Resolved Laser Photoacoustic Calorimetry

N2 Pumped Dye Laser

EM

Preamp

DSO

CFO NF cell

PAD

BS

N2 Pumped Dye Laser

EMEM

PreampPreamp

DSO

CFO NF cell

PAD

CFO NF cell

PAD

BS

Acoustic detector

laser

Photoacoustic Sound Waves

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Time (s)

No

rma

lize

d S

ign

al

(a.u

.)

Reference

Sample

)101( AlaserEkH

=1 for reference compound

Studies on M(CO)6(Cycloolefins), M = Cr, Mo, WW C O 5(olefin) O ptim ized G eom etries:W C O 5(olefin) O ptim ized G eom etries:

15.0

20.0

25.0

30.0

35.0

1 2 3 4 5 6 7 8 9

Number of Carbons

H o

r E

(kca

l/mo

l)

a)

5.0

15.0

25.0

35.0

45.0

55.0

65.0

1 2 3 4 5 6 7 8 9

Number of Carbons

Rin

g S

trai

n E

ner

gy (

kca

l/m

ol)

d)t

t

Metal-cycloolefins bond strengths correlate well but not exactly with the trend in ring strain energy

Electronic interactions, ring strain relief and Reorganizational effects: A molecular paradox

C=C Bond elongation

3.00

3.50

4.00

4.50

5.00

5.50

6.00

6.50

7.00

7.50

8.00

2 3 4 5 6 7 8 9

number of carbon atoms

∆ C

=C

(p

m)

trans

Changes in Pyramidalization Angle

5.0

10.0

15.0

20.0

25.0

30.0

2 3 4 5 6 7 8 9

number of carbon atoms

∆ p

yrim

idiz

atio

n an

gle

(deg

)

trans

Olefin Deformation Energy vs. Pyramidalization Angle

0.02.04.06.08.0

10.012.014.016.018.020.0

0.0 10.0 20.0 30.0

∆ pyrimidization angle (deg)

∆ E

(de

f.) (

kca

l/mo

l)

c8t c6

c8c

c5

c7

c4

c3

c2

Ring strain is relieved by the reorganization of the olefin as it binds, however, olefin reorganization is energetically costly, thus reducing the overall BDE.

Photodynamic Therapy (PDT)

Strong absorption in the red or near infrared (>630 nm) region of the spectrum.High quantum yield of triplet state to obtain large concentrations of the activated drug.High reactivity of the triplet with ground state oxygen to obtain measured high yields of active oxygen High affinity for diseased tissue against healthy one, to avoid the risk of photodestruction of healthy tissue Rapid metabolic rates so it can be excreted from the bodyLow toxicity in the darkSimple formulationFacile synthesis and modification of the structure.

The ideal photosensitizer:

Mechanisms of oxygen photosensitization

So

S1

To

3O2

1O2

Oxidation Reactions

Absorption

Fluorescence IC

ISC

Photoreaction

ēT-.

O2-.

Research Goal:To establish correlations between the yield of triplet sensitizer and active oxygen and the molecular structure of the photosensitizer. Correlations will lead to a rational design of sensitizers with optimized yields of triplet sensitizer and singlet oxygen

NovelPhotosensitizers (Prof. T.D. Lash)

Absorption Spectra

Emission Spectra: Fluorescence yieldTriplet energy

Triplet yield (PAC)

1O2 yield (Traps, TRF)

Computational Methods

N

NH N

HN

RR

R

R

R =Ph , C C Ph

N

NH N

HN

Et

Et

Et

Me

Et

Me

NXN

X = O, S, Se

N

N

HN

R = H, Me, t-But, Cl

OMe

MeO

R

R0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

300 400 500 600 700 800 900 1000

wavelength (nm)

Ab

so

rba

nc

e

Time

1 O2 E

mis

sion

Photoacoustic Sound Waves

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Time (s)

No

rma

lize

d S

ign

al

(a.u

.)

Reference

Sample

Experimental Techniques: Time Resolved VIS-NIR Emission Spectroscopy

Pumped N2

Dye Laser

CO NF cellBS

IRD

FEM

Preamp

F

PM

Pumped N2

Dye LaserPumped N2

Dye LaserPumped N2

Dye LaserN2

Dye Laser

CO NF cellBS

IRD

FEMEM

PreampPreamp

F

PM

Time

1 O2 E

mis

sion

@ 1

270

nm

kteICI 0

Extension of Conjugation, Distortion of Planarity and Photophysical Properties

TAP Emissions Spectrum

0

5

10

15

20

25

30

35

40

45

50

55

60

600 625 650 675 700 725 750 775 800 825 850 875 900 925 950 975 1000

Wavelength (nm)

Inte

nsi

ty

TAAP Emissions Spectrum

0

5

10

15

20

25

30

795 820 845 870 895 920 945 970 995 1020 1045

Wavelength (nm)

Inte

nsi

ty

TAAPAbsorption Spectrum

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Abs

TAP Absorption Spectrum

0

0.5

1

1.5

2

2.5

3

300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Ab

s

Extension of Conjugation, Distortion of Planarity and Photophysical Properties

TAAP

TAP

TPP

Research is a TEAM effort:

Collaborators: Tim Lash, Marge Jones, Pilar Mejia (ISU)Eric Weitz (Northwestern University)

Graduate Students: Richard Sniatynsky, Ken Kite, Darin Schlappi

Undergraduate Students: Hal Steiner, Jakoah BrgochPast: Cole Hexel, Paul Brackemeyer, Joel Eagles, Jeremy Woods,Tom Walczack, Ken Kite, Darin Schlappi.

High School Students: Casey Huftington, Delano Robinson, Julio Martinez.

Funding:Petroleum Research Fund – American Chemical Society.Project SEED – American Chemical Society.Illinois State University: Office of the Provost, College of Arts and Sciences, Department of Chemistry.