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