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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
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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
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O
OO
O
O
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O
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OO
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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
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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
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OO
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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
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OO
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OO
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O
O
OO
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O
O
O
O
O
O
Py
OHO
O
O
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OO
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OO
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O
O
OO
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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
34
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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