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Yue Qiu1, Lucas Antony2, Juan de Pablo2, Mark D. Ediger1
1. University of Wisconsin-Madison2. University of Chicago
Photochemical Stability of Glasses Can be Significantly Modulated by Molecular Packing
1
Glasses are widely used in modern technology
Photochemical stability is a concern in organic electronics.2
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Glasses are amorphous (non-crystalline) materials
• Molecularly smooth• Macroscopically
homogeneous
theodysseyonline.com/clarion/5-life-lessons-learned-while-making-cotton-candy/132073
Crystal packing can strongly influence photochemical reactivity, while glass packing has little effect
(tetrabenzoylethylene) furanone
3
Crystal case: Glass case:
Significant modulation of photo-reactivity in glasses has not been achieved until now.
Cohen and Schmidt, J. Chem. Soc. (1964)
α and β polymorphs have different photo-reactivity.
Royal, J.; Torkelson, J. Macromolecules 1992, 25, 4792-4796.
Photo-isomerization slightly differs by 5% after 100 hours aging.
~ 5%
Glasses can be made by cooling a liquid and physical vapor deposition (PVD)
4Ediger, M. D.; Angell, C. A.; Nagel, S. R. J. Phys. Chem. 1996, 100,13200-13212.
Substrate
P ~ 10-7
torr
Temperature-controlled
Heated Source
Physical vapor deposition
Properties of glasses depend on preparation methods.
Liquid-cooled glass
Volu
me
Temperature
Liquid
Supercooled Liquid
Glass 1
Glass 2
Crystal
Tg,2 Tg,1 Tm
Azobenzene photochemistry
• Azobenzene undergoes trans->cis isomerization
when irradiated by UV light.
• The reverse cis->trans isomerization can occur by
thermal relaxation.
Heat
5Merino, E.; Ribagorda, M. Beilstein J. Org. Chem. 2012, 8, 1071–1090.
PVD can prepare stable glasses of Disperse Orange 37 (DO37)
290 300 310 320-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
TSubstrate
/Tg = 0.88
% T
hick
ness
Cha
nge
Ramping Temperature (K)
Tg = 296 K
Tonset = 308 K
DO37
• PVD glass of DO37 shows increased density and enhanced kinetic stability.
• Density and onset temperature of DO37 glasses can be tuned by substrate temperature.6
220 240 260 280295
300
305
T onse
t (K)
Substrate Temperature (K)
Liquid-cooled Glass
LC Glass
T-gradient Substrate
Incr
ease
d ki
netic
sta
bilit
y
0.75 0.80 0.85 0.90 0.95 1.00TSubstrate / Tg
Photostability test: spectroscopic ellipsometry is used to measure sample thickness and birefringence during irradiation
Ellipsometrylight source
Defined input polarization
Measured output
polarization
DetectorPump beam�ૃ =532 nm
Sample
7
Simulation:
TSubstrate / Tg 0.76 0.82 0.85 0.91 0.97 LC Glass0.98
1.00
1.02
1.04
0 200 400 600 800 1000
-0.3
-0.2
-0.1
0.0
(
LC)
a
# of IterationsO
rder
Par
amet
er
b
PVD glasses show significantly enhanced photo-stability
8
Experiment:
0.99
1.00
1.01
0 2000 4000 6000
-0.02
0.00
0.02
TSubstrate/Tg 0.82 0.85 0.88 0.91 0.94 0.98 LC Glass
# of Photons Absorbed Per Molecule
(
LC)
a
Time (s)
Shift
ed n
z-nx
b
0 300 600 900
Photo-stability of DO37 PVD glasses correlates with density
• PVD glass can be nearly 2 orders of magnitude more photo-stable than liquid-cooled glass.
• Glasses with higher density are more photo-stable.
9
Photostability of glasses is significantly modulated by density of packing.
101
102
103
101
102
103
240 260 280 300 314.6
0.00.40.81.2
t =0
.1%
(s)
a t n=0
.005
(s)Photochemical Stability
(A
D)/
(LC
)10
0
TSubstrate (K)
bTg
Density
LC Glass
0.80 0.85 0.90 0.95 1.00 1.05TSubstrate / Tg
Experiment:
Discussion: The activation energy barrier for stable glass prevents photo-isomerization reaction
10
Stable glasses have higher activation free energy for a successful photo-isomerization reaction.
TSubstrate / Tg 0.76 0.82 0.85 0.91 0.97 LC Glass
0 200 400 600 800 1000
0.0
0.2
0.4
0.6
Phot
oiso
mer
izat
ion
(cis
frac
tion)
# of Iterations
c
trans
cis
S1
S01. 5
32 n
m e
xcita
tion
2. Rapid relaxation3
Dihedral Angle (θ)0 180
4
Ene
rgy Stable
glass
Simulation
Summary
11
3. Photo-stability of stable glasses might be generally important to other organic systems, and adds insight to organic electronics design.
1. Stable glass can be significantly more photo-chemically stable than liquid-cooled glass. And photo-stability correlates with density.
101
102
103
101
102
103
240 260 280 300 314.6
0.00.40.81.2
t =0
.1%
(s)
a t n=0
.005
(s)
Photochemical Stability
(A
D)/
(LC)
100
TSubstrate (K)
bTg
Density
LC Glass
2. Stable glass is more photo-stable because it prevents the photo-isomerization reaction.
23 July 2013, SPIE Newsroom. DOI: 10.1117/2.1201307.005007
Acknowledgement
12
University of Wisconsin-MadisonProf. Mark D. EdigerProf. Lian Yu
University of ChicagoLucas AntonyProf. Juan de Pablo
FundingNSF DMREF
13
Supporting slides
14
Birefringence definitiondn=nz-nx
nz
nx
Biaxial model:
ny
15
Simulated Excitation1. Excitation selection:
• randomly select a molecule (blue)
• accept or reject for excitation with probability cos2(θ)
• continue until 1% selected (red)
2. Switch selection to the cis potential
3. Run 100000 steps
4. Return all molecules to trans potential
5. Repeat
16
Photo-stability by simulation show similar trend as experimental results.
Excitation iteration required for (a) 0.7% density change, (b) 0.005 order parameter change.
• Simulation shows similar trend as experiments. Glasses with higher density are more photo-stable.
• PVD glass is about 1 order of magnitude more photo-stable than liquid-cooled glass.
101
102
101
102
0.75 0.80 0.85 0.90 0.95 1.012
0
2
4
Itera
tions
=
0.7%
a
Photochemical Stability
Itera
tions
S=
0.00
5 TSubstrate / Tg
(
AD
)/(L
C)
100
b
LC Glass
Density