Jamie Yip, Jean Duhamel in cooperation with Alex Adronov, Greg Bahun

Chemistry Department Chemistry DepartmentUniversity of Waterloo McMaster University

IPR SymposiumMay 1, 2009

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IntroductionBackgroundExperimental ResultsConclusionsAcknowledgements

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What are dendrimers?◦ Dendron – tree◦ Meros – part◦ Molecules with a

tree-like structure!

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Sheiko, S. S.; Gauthier, M.; Moller, M. Macromolecules. 1997, 30, 2343-2349.

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Why study dendrimers?◦ Molecular storehouse

Within internal cavitiesAt end-groups

4

Tekade, R. K.; Kumar, V. P.; Jain, N. K. Chem. Rev. 2009, 109, 49-87.

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How are we going to study dendrimers?◦ Fluorescence Dynamic Quenching (FDQ)

experimentsProvides information concerning the flexibility of our molecules

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Fluorescence Dynamic Quenching (FDQ)◦ Involves monitoring quenching of a chromophore

(M) by its quencher (Q)

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k1[Q]loc = kqM* + Q

1/τM

M Qhν + M + Q

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Pyrene◦ High molar extinction coefficient

ε = 43,000 M-1cm-1 for 1-pyrenebutanol in ethanol

◦ High fluorescence quantum yieldΦ = 0.32 in cyclohexane

◦ Long natural lifetimeτM = 210 ns for 1-pyrenebutyric acid in THF

◦ Collisional quenchingSelf-quenches upon encounter of an excited pyrene with a ground-state pyrene, forming excimer

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k1[Py]loc= kq

Pyrene◦ The Birks’ Scheme

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hν + Py + Py (PyPy)*Py*+ Py

1/τM 1/τE

k-1

Py PyPyPyPyPy

Py PyPyPyPyPy

Py PyPyPyPyPy

Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970; p 301-307.

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OHO

O

O

O

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

O

O

OO

OO

OO

OHO

O OOO

OHO

OO

OO

OO

OO

OO

OO

OHO

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O O N

n

Dendrimer Structure◦ Four generations studied (G1-G4) as well as four

linear polystyrene/dendron hybrids (G1PS-G4PS)

9G1G2G3

G4G*GPS

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Experimental Techniques◦ UV-Visible Spectroscopy◦ Steady-State Fluorometry (SSF)◦ Time-Correlated Single Photon Counting (TC-SPC)◦ Gel Permeation Chromatography (GPC) with

fluorescence detector

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UV-Visible Spectroscopy◦ Absorbance measurements◦ Beer-Lambert Law:

= absorbance= molar extinction coefficient of pyrene (M-1cm-1)= concentration of absorbing species (M)= path length between incident and transmitted light (cm)

◦ Maintain a low pyrene concentration to ensure no intermolecular quenching is observed

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clA ε=Aεcl IP

R 2009

Steady-State Fluorometry◦ Provides steady-state emission spectra

Maintain constant excitation wavelength, monitor emission intensity over a range of wavelengths

◦ Can determine the IE/IM ratio

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G1 in THF[Py] = 2.5 μMλex = 344 nm

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Steady-State Fluorometry

= fluorometer instrument constant, = fluorescence quantum yield of excimer and

monomer, respectively= monomer fluorescence lifetime

0Eφ

Mτ

κ

k1[Py]loc= kq

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IM

IE

G1 in THF[Py] = 2.5 μMλex = 344 nm

hν + Py + Py (PyPy)*Py*+ Pyk-1

[ ]locMM

E

M

E PykII

10

0

τφφκ=

Cuniberti, C.; Perico, A. Eur. Polym. J. 1980, 16, 887-893.

0Mφ IP

R 2009

Time-Correlated Single Photon Counting◦ Allows acquisition of fluorescence decays at given

excitation and emission wavelengths◦ Since pyrene monomer and excimer fluoresce at

distinct wavelengths, monomer and excimer decays can be acquired separately◦ Can determine kq, the quenching rate constant

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Time-Correlated Single Photon Counting

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G4 in THF[Py] = 2.5 μMλex = 344 nm

Monomer, λem = 375 nm Excimer, λem = 510 nm

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Time-Correlated Single Photon Counting

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G4 in THF[Py] = 2.5 μMλex = 344 nm

Monomer, λem = 375 nm Excimer, λem = 510 nm

321321)( τττ

ttteAeAeAtI−−−

++=

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Time-Correlated Single Photon Counting◦ From global analysis:

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τ1 (ns) A1 τ2 (ns) A2 τ3 (ns) A3 χ2 <τ> (ns) <k> (107 s-1)1.19 0.83 2.46 0.13 25.1 0.014 1.11 1.70 58.2

∑

∑

=

==>< n

ii

n

iii

A

A

1

1τ

τ

locqM

Pykkk ][111==−

><=><

ττ

nsM 210=τ

Winnik, M. A.; Egan, L. S.; Tencer, M. Polymer. 1987, 28, 1553-1560.

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0

200

400

600

800

1000

1200

350 400 450 500 550 600

Inte

nsity

(A.U

.)

Wavelength (nm)

0

400

800

1200

1600

2000

350 400 450 500 550 600In

tens

ity (A

.U.)

Wavelength (nm)

0

200

400

600

800

1000

1200

350 400 450 500 550 600

Inte

nsity

(A.U

.)

Wavelength (nm)

Steady-State Fluorometry

◦ Is the decrease due to increased dendritic arm stiffness?

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Solvent = THF[Py] = 2.5 μMλex = 344 nm

G Series GPS Series

G1

G3G2G4

G1PS

G3PSG2PS

G4PS

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Time-Resolved Fluorescence Decays

◦ Recall: IE/IM should be proportional to <k>!

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0

10

20

30

40

50

60

70

0 1 2 3 4 5

<k>

(107

s-1)

Generation Number

0

10

20

30

40

50

60

70

0 1 2 3 4 5<k

> (1

07s-1

)Generation Number

G Series GPS Series

><= kII

MM

E

M

E τφφκ 0

0

Cuniberti, C.; Perico, A. Eur. Polym. J. 1980, 16, 887-893.

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IE/IM and <k>

◦ Why the decrease in IE/IM ratio for G4?

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0

2

4

6

8

10

0

10

20

30

40

50

60

70

0 1 2 3 4 5IE /IM

<k>

(107

s-1)

Generation Number

0

4

8

12

16

20

0

10

20

30

40

50

60

70

0 1 2 3 4 5

IE /IM

<k>

(107

s-1)

Generation Number

G Series GPS Series□ - <k>◊ - IE/IM

□ - <k>◊ - IE/IM

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Contributions from τM from monomer decays

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Sample fMfree (% of total contribution)

G1 3.3G2 0.62G3 0.29G4 3.1

G1PS 1.5G2PS 0.61G3PS 0.08G4PS 0.23

OHO

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

O OO

O

O

O

O

O

O

O

OHO

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The new IE/IM equation◦ <k> ignores contributions from free monomers in

solution.◦ The new IE/IM equation takes into account all

species in solution, as well as their relative contributions, based on fluorescence decay data.

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The new IE/IM equation

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0

2

4

6

8

10

0

5

10

15

20

0 1 2 3 4 5

IE /IM

Cal

cula

ted

I E/I

M

Generation Number

0

4

8

12

16

20

0

5

10

15

20

25

30

0 1 2 3 4 5

IE /IM

Cal

cula

ted

I E/I

M

Generation Number

G Series GPS Series

□ - Calculated IE/IM◊ - Actual IE/IM

□ - Calculated IE/IM◊ - Actual IE/IMIP

R 2009

Hmm?OHO

Gel Permeation Chromatography

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G4 in THF[Py] = 25 μMλex = 344 nm

Excimer, λem = 510 nm

Monomer, λem = 375 nm

Py-butyric Acid, λem = 375 nm

OHO

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

G4

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G4 Purification

The second peak fluoresced with a lifetime of 210 ns!

25

0

5

10

15

20

25

01020304050607080

0 1 2 3 4 5

IE /IM

<k>

(107

s-1)

Generation Number

0

500

1000

1500

2000

2500

350 400 450 500 550 600

Inte

nsity

(A.U

.)

Wavelength (nm)

*

G1

G2G3

G4

Purified G4

□ - <k>◊ - IE/IM

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Molecular Mechanics Optimizations◦ Recall:

As generation number increases, <k> increases.Previous studies have shown that, at low generations, k1 decreases slightly with increasing generation number.Thus, [Py]loc must increase.Assuming k1 constant, [Py]loc should show similar trends to <k>

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locPykk ][1=><

Meltzer et. al. Macromolecules. 1992, 25, 4541-4548.

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Scenarios to consider:◦ Pyrene is free to diffuse throughout

◦ Pyrene is confined to the outer shell of the dendrimer

◦ A combination of the twoNeed to determine the volume occupied by pyrene

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OHO

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

Py

OHO

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

OO

O

O

O

O

O

OO

OO

OO

O

O

O

O

O

O

Py

dE-E

dc

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Scenario 1:◦ Pyrene free to diffuse throughout

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dE-E = 23 Å

dE-E = 33 Å

dE-E = 42 Å

dE-E = 51 Å

3

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12][EE

n

loc rPy

−

−=

π

0.0

0.1

0.2

0.3

0.4

0

10

20

30

40

50

60

0 1 2 3 4 5

[Py]loc (mol·L

-1)<k>

(107

s-1)

Generation Number

□ - <k>◊ - [Py]loc

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0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

10

20

30

40

50

60

0 1 2 3 4 5

[Py]loc (mol·L-1)

<k>

(107

s-1)

Generation Number

Scenario 2: Pyrene excluded to outer shell

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( )33

34

12][cEE

n

loc rrPy

−−

=−π dc = 17 Å

dc = 26 Å

dc = 36 Å

dc = 45 Å

□ - <k>◊ - [Py]loc

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Scenario 3:◦ G1 is free to diffuse throughout, higher generations

are confined◦ Calculate k1 for G1◦ Assume k1 is constant for all generations◦ Calculate corresponding rc

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Scenario 3◦ For G1:

◦ For G2-G4:

◦ Result: For G2-G4, rc corresponded to pyrene attachment point minus 1-2 carbons!

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locPykk][1><

=

1

][kkPy loc><

=

31

3

][34

12⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −−= −

loc

n

EEc Pyrr

π

rc

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The decrease in IE/IM ratio is due to residual pyrenebutyric acidAt generations > 1, internal steric hindrance may exclude pyrene to the outer spherical shell defined by the arms of the dendrimer molecules

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Dr. Jean DuhamelDr. Alex AdronovGreg BahunThe Duhamel Group

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